Somewhere in the sediments and rocks beneath the ocean floor, it gets too hot for living things. But how far down? Even after drilling kilometers into the ocean floor, scientists have found that microbes persist. “We keep digging and digging and digging deeper and have not hit the bottom of the biosphere,” says Jan Amend, a geochemist at the University of Southern California in Los Angeles.

A new ocean drilling expedition will try to settle the question by drilling into crust where high temperatures are found unusually close to the sea floor, bringing life’s thermal limit within reach. On 13 September, the research vessel Chikyu will set sail from Shimizu, Japan, and sink its bits into a patch of ocean floor where the sediments should reach 130°C at the maximum drill depth of 1260 meters. Somewhere along the way, the team expects, life should succumb to rising temperatures.