Which incident that occurred on Earth about 40 thousand years ago is called the "Lashamps event"?

The Laschamps event was a geomagnetic excursion (a short reversal of the Earth's magnetic field). It occurred 41,400 years ago, during the end of the Last Glacial Period. It is known from geomagnetic anomalies discovered in the 1960s in the Laschamps lava flows in Clermont-Ferrand, France.

The Laschamps event was the first known geomagnetic excursion and remains the most thoroughly studied among the known geomagnetic excursions.

The magnetic excursion has since been demonstrated in geological archives from many parts of the world. The magnetic field was reversed for approximately 440 years, with the transition from the normal field lasting approximately 250 years. The reversed field was 75% weaker, whereas the strength dropped to only 5% of the current strength during the transition. This reduction in geomagnetic field strength resulted in more cosmic rays reaching the Earth, causing greater production of the cosmogenic isotopes beryllium 10 and carbon 14.

The Australian Research Council is funding research to analyze a kauri tree uncovered in New Zealand in 2019. According to its carbon-dating, the tree was alive during the event.

The geomagnetic field was at low levels from 42,200–41,500 years ago. This period of low magnetic field has been termed the Adams Event or Adams Transitional Geomagnetic Event, a tribute to science fiction writer Douglas Adams, who wrote in The Hitchhiker's Guide to the Galaxy that "42" was the answer to life, the universe, and everything.