Every now and then (on a scale of hundreds of thousands of years), Earth’s magnetic field does a flip-flop: Magnetic north becomes magnetic south and vice versa. In between these major realignments, the magnetic poles wander over shorter distances, throwing models of Earth’s magnetic field off kilter. A record of these polar wanderings, the paleomagnetic timescale, is preserved in successive layers of volcanic and sedimentary rocks. Scientists take great pains to ensure that they interpret this record accurately. Unfortunately, they have not been as careful with their spelling.
Occasional shorter polarity reversals or large perturbations of the magnetic field are called excursions, events, or microchrons [Laj and Channell, 2007; Roberts, 2008]. The first evidence of such a geomagnetic excursion was identified by Bonhommet and Babkine  in lava and volcanic features from the Chaîne des Puys, a volcanic chain near the village of Laschamps in the Auvergne region of central France.
Signs of this reversal have since been observed in volcanic and sedimentary rocks in many places around the world, and the event has, logically, been incorporated into paleomagnetic timescales as the Laschamp excursion. The most recent—and we hope the most accurate—dating by Guillou et al.  indicated that the excursion occurred about 40,000 years ago; its duration was probably less than 2,000 years [Laj and Channell, 2007; Roberts, 2008].
During the event, the virtual geomagnetic pole traveled a clockwise loop around Earth. When it was oriented southward, the magnetic field was at its minimum, about one fifth to one sixth of the present magnetic field [Roperch et al., 1988; Chauvin et al., 1989; Laj et al., 2004]. This minimum is well correlated with an increase in the beryllium isotope ratio 10Be/9Be in oceanic sediments [Laj and Channell, 2007; Ménabréaz et al., 2012]. This relative increase in 10Be isotope levels reveals a general decrease in the magnetic shielding of Earth from cosmic rays during the excursion (which might have been one cause for the extinction of the Neanderthals [Valet and Valladas, 2010]).
It is, however, unfortunate that such an important event in Earth’s history is misspelled in all scientific documents. The correct name is Laschamps, instead of Laschamp. We would like to explain the origin of the correct spelling.
How Laschamps Got Its s
Most cartographers since Cassinis (in 1777) have curiously forgotten the final s of Laschamps, except for Michelin’s road map number 32 (1909–1929). Current signage and the local ancestral use attest to the correct spelling Laschamps, and it appeared in this form in the Puy-de-Dôme localities dictionaries [Bouillet, 1854; Tardieu, 1877]. The Gordon Bennett car race cup in 1905 has popularized this spelling internationally.
As for the etymology of the name of the village, Jean-Pierre Chambon, a specialist in Roman topology, gave a formal explanation in a personal communication:
Laschamps consists of the definite feminine plural article las associated with the substantive chalm (plateau or moor) in the plural; the substantive was altered to champ, a very frequent evolution in toponymy. The Laschamps current form is the direct legacy of the era (14th or 15th century) during which the French language borrowed its toponymy from the medieval Occitan.
Thus, it would be desirable for paleomagneticians to get into the habit of restoring the final s to the name of such a significant geomagnetic excursion.
We owe many thanks to Jean-Pierre Chambon for invaluable information about the medieval Occitan language. The authors are also very grateful to two anonymous reviewers for their constructive remarks.