Hal Clement - Still River

The Riddle of Enigma Years of exacting study were coming to an end. All five student scientists, each the best its species had to offer, were one step away from their final degrees. One examination remained: the students were to determine why the tiny planet of Enigma, a planet with so little mass that it should never have had an atmosphere, in fact it did. Everything seemed to be running smoothly until the only Human member of the team, Molly was accidentally blown into one of the deep fissures that pocked the planet's surface. She found herself trapped in a series of tunnels and caves that only led her deeper into the riddle that was at the heart of Enigma. But as her concerned companions followed her, they found Enigma's most puzzling anomaly. For in the dark passages they discovered life - life which couldn't possibly exist.