The morning of 23 July 2024 was like most summer mornings in Yellowstone National Park. Cars vied for parking spaces, bison lounged in meadows, and tourists strolled along boardwalks taking in sights of bubbling springs and spouting plumes of water and steam. All were unaware of the pressure that had built underneath Black Diamond Pool, a thermal spring in Wyoming’s Biscuit Basin about 3.5 kilometers northwest of famed Old Faithful Geyser.
Suddenly, just before 10:00 a.m., jets of muddy, rock-laden water and steam shot from the turbid depths of the pool, building into bursts as high as 400–600 feet (~120–180 meters) that showered the surrounding area and boardwalk with rocks and mud. Water from the pool surged toward the nearby Firehole River, carrying boulders and debris, and a steam plume was visible from kilometers away.
Fifty-five seconds after the explosion began, it was over. Left behind was a roughly 1-square-kilometer debris field, as well as many stunned onlookers, fresh from scrambling away from the unexpected outburst and wondering what had just happened.
Safety was the paramount concern in the immediate aftermath of the event. But scientists also mobilized quickly to monitor for additional activity and to collect timely observations that could help piece together why the explosion happened. On a longer timescale, federal, state, and academic organizations are working together to better understand the dynamics and triggers of hydrothermal explosions to recognize warning signs of future events before they occur.
What Are Hydrothermal Explosions?
Hydrothermal explosions, like the July 2024 event at Black Diamond Pool (Figure 1), occur in many volcanic-hydrothermal areas around the world. When a pressurized hot water reservoir instantaneously decompresses, superheated water violently flashes to steam that has enough energy to break surrounding rock [Muffler et al., 1971; Thiéry and Mercury, 2009; Browne and Lawless, 2001; Montanaro et al., 2022].
Such explosions exist on a continuum from large, destructive events to smaller geyser eruptions that routinely spew water and steam into the air. Geysers are caused by constrictions in hydrothermal plumbing systems that temporarily trap boiling water and steam until the pressure is high enough for the water to erupt. Hydrothermal explosions, on the other hand, occur less frequently. They are primed by the gradual pressure increase in a confined system, followed by rapid decompression caused either by rupturing of a hydrothermal seal or by an external event like a landslide or earthquake. Large geyser eruptions can destroy geysers’ plumbing systems and throw rock and mud like hydrothermal explosions, and small, spontaneously reoccurring hydrothermal explosions may arguably be more consistent with geyser activity.
In Yellowstone National Park, at least 18 massive hydrothermal explosions have formed craters ranging from 300 to 2,500 meters across.
In Yellowstone National Park, the occurrence of hydrothermal explosions has been recognized for decades. Since the end of the most recent glaciation roughly 14,000 years ago, at least 18 massive hydrothermal explosions have formed craters ranging from 300 to 2,500 meters across, the largest of which—formed about 13,800 years ago—is the biggest explosion crater on Earth [Muffler et al., 1971; Morgan et al., 2009; Christiansen et al., 2007].
More than 2 dozen hydrothermal explosions have been documented within Yellowstone National Park since its founding in 1872 [Christiansen et al., 2007]. One of the best-observed events prior to 2024 was the explosion of Porkchop Geyser in Norris Geyser Basin on 5 September 1989 [Fournier et al., 1991]. That event—witnessed by nine people, none of whom were injured—threw small rocks and debris 60 meters from the vent and left a crater more than 10 meters wide.
Smaller hydrothermal explosions occur more frequently than larger ones (maybe as often as annually), but they usually go unwitnessed because they occur in the backcountry, at night, or during winter months. Hydrothermally active areas around the world sometimes show signs of instability or increases in temperature prior to an explosion; however, there are no known universal precursory signals upon which forecasts can be based.
Keeping Watch over Yellowstone’s Activity
The lack of knowledge about hydrothermal explosion occurrence rates, precursory signals, and triggers motivated the Yellowstone Volcano Observatory (YVO), a consortium of nine federal, state, and academic organizations, to include hydrothermal processes and hazards in its recently developed hazards monitoring plan [Yellowstone Volcano Observatory, 2022]. The plan includes the installation of broadband seismic, infrasound, thermal, and deformation sensors within geyser basins to better detect anomalous hydrothermal activity and investigate the potential to forecast hazardous events.
A prototype hydrothermal monitoring station, installed in Norris Geyser Basin in 2023, immediately paid dividends. The station clearly detected infrasound signals from nearby geyser eruptions and a small hydrothermal explosion that occurred on 15 April 2024—the first hydrothermal explosion in Yellowstone National Park to be documented by instrumental monitoring [Poland et al., 2025].
Yellowstone Volcano Observatory’s volcano monitoring network barely detected the explosion, even though it was big enough to destroy a section of boardwalk adjacent to Black Diamond Pool.
However, no hydrothermal monitoring station was installed at Biscuit Basin in July 2024, and YVO’s volcano monitoring network barely detected the explosion, even though it was big enough to destroy a section of boardwalk adjacent to Black Diamond Pool. The destructive event—thankfully, no injuries resulted—emphasizes the importance of expanded monitoring in geyser basins of Yellowstone National Park. It also highlights the risk posed by even small explosions that occur when people are nearby.
Much remains unknown about the processes leading to hydrothermal explosions and how best to safeguard the more than 4 million visitors to Yellowstone National Park every year from this underappreciated hazard [e.g., Montanaro et al., 2022]. The goal of postexplosion scientific investigations is to develop understanding that will enable better monitoring, detection, and, potentially, forecasting of future dangerous hydrothermal events.
Black Diamond Pool’s Explosive Past and Present
Explosive activity has recurred sporadically at Black Diamond Pool over its roughly 120-year life. Broken, angular rocks from previous explosions that were cemented back together before being ejected on 23 July 2024 provide evidence of this repeated explosive activity.
According to early geologic maps and photographs, Black Diamond Pool did not exist before 1902. It likely formed dramatically from a hydrothermal explosion sometime between then and 1912. Documents preserved in the Yellowstone National Park archives reference a few short periods of explosive activity that enlarged the new pool and formed two additional springs.
The area was quiet after 1960 until a series of short explosive events of varying intensity (though none approaching the scale of the July 2024 event) reinitiated in 2006. The frequent activity ceased by early 2013, and only three isolated events were reported between then and July 2024.
The 23 July hydrothermal explosion—which occurred during the park’s busiest month—stunned tourists, National Park Service (NPS) officials, and the scientific community. Visitation numbers had peaked a few days earlier, and on the day of the explosion, cameras recorded 209 visitors to Biscuit Basin by 9:00 a.m. Within minutes of the approximately 10:00 a.m. event, law enforcement rangers arrived on the scene and quickly closed the basin to the public to prevent injuries should explosive activity continue.
YVO’s initial response primarily involved communicating to the public and emergency managers about the cause of the event and the potential for additional activity. Observatory scientists also fielded numerous media inquiries.
Coordination of the scientific response began in parallel with these communications activities. YVO scientists and experienced collaborators from other institutions deployed to the field within hours to days to install monitoring equipment and gather time-sensitive data using a variety of approaches.
Fanning Out in Biscuit Basin
Working near an unstable, potentially explosive pool in the immediate aftermath of the explosion was an exercise in situational awareness.
Working near an unstable, potentially explosive pool in the immediate aftermath of the explosion was an exercise in situational awareness, but the extensive training and experience of the scientists involved helped to ensure their safety.
Field teams worked in pairs, with one person keeping an eye out for signs of an ensuing explosion while the other collected data. High-temperature areas surrounding the pool suggested the presence of boiling water or steam underneath a thin crust where the ground could easily collapse or another explosion could break out. Near the pool edge, slippery mud and overhangs that could crumble unexpectedly into the pool also posed particular hazards.
The field teams also knew that newly unsealed hydrothermal systems can emit higher-than-normal amounts of hazardous gases. Thankfully, blowing winds following the explosion diluted potentially dangerous concentrations as well as the strong perfume of acid, sulfur, and hydrocarbons, helping the teams get on with their work.
Geology and Mapping. Hydrothermal explosions leave behind debris fields that can be used to discern many properties of the explosions [Breard et al., 2014]. For example, the size and distribution of ballistic blocks around the vent provide clues about the energy of the explosion. This information also enables calculations of ballistic vulnerability—the probability of a human fatality at any given location around the vent in the event of another explosion of similar size. In addition, rocks excavated from the preexisting subsurface hydrothermal system are useful for understanding the pressure and temperature conditions before the explosion and how sealed the system was.
Field teams from Colorado State University, NPS, and the University of California, Berkeley documented the sizes, distribution, and lithology of ballistic blocks thrown by the explosion to begin piecing together what the underlying hydrothermal system looked like before and during the explosion. NPS also used an uncrewed aerial vehicle to collect thermal and structure-from-motion imagery of the deposits and the surrounding area. These images helped identify areas with elevated temperatures and quantify the volume of material ejected by the 23 July explosion.
Dense seismic networks, which can sense the vibrations of bubbles and the brecciation of rock, are powerful tools for resolving subsurface hydrothermal plumbing and detecting small explosions.
Near-Surface Geophysics. Dense seismic networks, which can sense the vibrations of bubbles and the brecciation of rock, are powerful tools for resolving subsurface hydrothermal plumbing, detecting small explosions, and helping scientists assess hazards from ongoing activity.
To record seismic signals in the aftermath of the explosion at Black Diamond Pool, the University of Utah deployed a temporary array of 33 seismometers around the pool by 26 July, and the instruments recorded for about 2 months. Four infrasound microphones were also deployed roughly 300 meters northwest of the pool from 19 August to 18 October. These data will be processed to pinpoint and explore signals from geyser activity and subsequent small hydrothermal explosions in Biscuit Basin.
Several weeks after the explosion, field teams from the University of Wyoming and NPS collected nuclear magnetic resonance (NMR), electrical resistivity (ER), and transient electromagnetics (TEM) datasets. NMR data provide estimates of the volume and location of water stored in the subsurface, including in confining, low-permeability zones. ER, which measures resistivity encountered by electrical currents, is ideal for identifying water-saturated subsurface pathways, as hydrothermal waters contain dissolved salts and are electrically conductive. TEM uses pulses of electric current to induce electric and magnetic fields underground. How fast these fields decay is another indication of variations in subsurface resistivity.
Together these techniques paint 3D views of hydrothermal fluids and lithological contrasts in the subsurface—important information for understanding the conditions and characterizing hazard potential in the postexplosion Black Diamond Pool system.
Water and Gas Chemistry. Gas emissions and water chemistry data were collected after the 23 July explosion by the U.S. Geological Survey (USGS), Montana Technological University, and the University of Wyoming to help probe underground processes.
Gridded measurements of carbon dioxide gas efflux, for example, provide information on spatial variations in diffuse gas fluxes at the surface that can be used to map subsurface gas pathways. Simultaneous measurements of isotopes of the short-lived radioactive gas radon in the same samples used for carbon dioxide measurements can help identify the sources of emissions and timescales of gas movement.
The chemical composition of the water in Black Diamond Pool is important because the solubility of different chemical species depends on the temperature at which water and rocks react. Critically, silica solubility decreases with decreasing temperature, and as hydrothermal waters cool, amorphous (noncrystalline) silica precipitates in subsurface flow paths [Fournier, 1985].
Past analyses of water chemistry at Black Diamond Pool have indicated that water and rocks there react at lower temperatures compared with systems farther south in Upper Geyser Basin, including at Old Faithful [Price et al., 2024]. These lower temperatures are more favorable for silica precipitation and may contribute to sealing flow paths and building pressure for hydrothermal explosions in Biscuit Basin.
Early Insights into the 2024 Explosion
The data collected following the explosion of Black Diamond Pool on 23 July 2024 are still being analyzed to provide a detailed account of the conditions preceding and following the event. However, some preliminary insights are available from the initial observations.
Many indicators point to the explosion being caused by self-sealing in the hydrothermal system.
Many indicators point to the explosion being caused by self-sealing in the hydrothermal system, with the result that increases in subsurface pressure eventually overcame the strength of the sealing rocks—a common mechanism for hydrothermal explosions globally [Morgan et al., 2009; Montanaro et al., 2022]. The lack of a strong earthquake nearby, either before or during the explosion, indicates it was not seismically triggered.
Furthermore, some of the ejected debris—namely, minimally altered, high-porosity, and high-permeability conglomerates and sandstones—likely contained much of the liquid water that flashed to steam and powered the explosion (Figure 2). On the other hand, completely silicified and intensely altered low-permeability rocks also found in the debris field likely constituted the seal that contained the pent-up pressure before the explosion.
The initial analyses of the seismic and infrasound data, as well as observations from scientists and passing visitors, indicate that small explosions at Black Diamond Pool have continued since 23 July 2024 through to the present, posing an ongoing hazard. Some of these explosions have been accompanied by water surges flowing east into the nearby Firehole River and have been large enough to carry seismic instruments several meters downhill and partially bury others in fine sediment. Two witnessed events were observed to throw water, mud, and small rocks 20–30 feet (6–9 meters) into the air. A webcam installed in mid-May 2025 to better document activity at Black Diamond Pool captured a similar small eruption on 31 May 2025.
Better Science for Better Response
New hazard maps and recent geophysical investigations will guide the National Park Service’s response to ensure public safety within Biscuit Basin.
The scientific response to the 23 July 2024 hydrothermal explosion has focused on improving understanding of the event to inform strategies that can be used to detect, and potentially forecast, similar future explosions. New hazard maps and recent geophysical investigations will guide NPS’s response to ensure public safety within Biscuit Basin, helping to address specific questions such as when the basin can be reopened, whether walkways must be relocated, and what the short-term probability of another large explosive event at Black Diamond Pool is. Scientific investigation will also guide YVO’s efforts to deploy targeted monitoring to other hydrothermal areas in Yellowstone National Park.
Hydrothermal explosions in Yellowstone National Park are an underappreciated hazard, and a pressing need exists to better understand where, why, and how often they happen. Filling these knowledge gaps requires multidisciplinary studies that consortia like YVO and its collaborators are well suited to undertake. Ultimately, improved monitoring of hydrothermal hazards will aid risk assessment and mitigation and help park officials and visitors avoid dangerous situations in Biscuit Basin, elsewhere in Yellowstone National Park, and at hydrothermal systems worldwide.
Acknowledgments
We especially thank Jamie Farrell, who assisted with preparation of this article and led the deployment of temporary seismometers and infrasound arrays in Biscuit Basin after the July 2024 explosion. We also acknowledge the many people involved in event response, scientific investigation, and management and policy decisions associated with the 23 July 2024 explosion of Black Diamond Pool. Scientists and personnel from USGS, NPS, Colorado State University, the University of Utah, the University of Wyoming, and Montana Technological University who have contributed include Phillip Kondracki, Alex Hammerstrom, Kiernan Folz-Donahue, Elle Blom, Blaine McCleskey, Sara Peek, Shaul Hurwitz, Steven Rice, Carrie Guiles, Jaclyn Mcllwain, Hillary Robinson, Andy Parkinson, Lexi Peterson, Lisa Morgan, Pat Shanks, Greg Vaughan, Jen Lewicki, Alycia Cox, Michael Loya, Andrew Miller, Katie Copeland, Kallen Snow, and Adaeze Ugwu. We thank Shaul Hurwitz and Patrick Muffler for constructive reviews. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.
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Author Information
Lauren Harrison ([email protected]), Colorado State University, Fort Collins; Michael Poland, Yellowstone Volcano Observatory, U.S. Geological Survey, Vancouver, Wash.; Mara Reed, University of California, Berkeley; Ken Sims, University of Wyoming, Laramie; and Jefferson D. G. Hungerford, National Park Service, Mammoth, Wyo.