Caves near Lake Pepin were once mined for key ingredient for gun powder.

The University of Minnesota researcher was the first to rediscover caves French explorers abandoned in the 1700s. He began exploring and categorizing the caves near Lake Pepin in 2004 and is defending his doctoral thesis on the research this semester.

The French explorers, including Pierre-Charles Le Sueur, mined the caves for saltpeter, the main ingredient in gunpowder until World War I.

Historians have unearthed the remains of caves containing saltpeter in the southern parts of the U.S., but they never made it this far north. And the explorers never mapped the saltpeter caves in Minnesota.

So Brick had to sift through Le Sueur’s notes and journals describing the area to find and map the caves himself.

via MN Daily.