These Cave Rocks Are Made out of Bacteria

Stalactites hold tight to the ceiling, the saying goes, and stalagmites might grow high enough to reach it. But the simple mnemonic doesn't come close to covering the variety of weird, rocky shapes growing all over a cave. There are even, it turns out, rocks made from bacteria. They're not putting the "tight" in "stalactite" so much as the "ack!" Researchers found the microbe-made rocks in a cave in northern Sweden called Tjuv Antes. (It's named for a fugitive thief who allegedly hid there in the 1800s.) Therese Sallstedt and Magnus Ivarsson of the University of Southern Denmark's Nordic Center for Earth Evolution, along with colleagues from Sweden and Spain, saw many kinds of rocky formations in the cave. There were smooth crusts of stone. There were bubbly structures that looked like popcorn. There were structures with delicate fingers like coral (above). The walls of Tjuv Antes Cave are made ...