After a long absence, veteran Clement (Mission of Gravity; Cycle of Fire, etc.) returns with a yarn about a group of students--one human, four aliens--who, as part of their quest for a higher degree, must explore and report on the peculiar planet Enigma 88. Enigma is a body too small to retain an atmosphere, and yet it does--albeit a poisonous one; the task for human Molly and her alien companions is to find out why. During their investigations, Molly falls down a hole and discovers that the planet is honeycombed with caverns and tunnels, complete with subterranean ammonia rivers and lakes, and--lifeforms. (It emerges that the latter were brought unwittingly by previous expeditions.) Molly gets thoroughly lost; as the other team members search for her, various setbacks and injuries occur; but the data the students are seeking slowly accumulates. As usual, Clement's scientific puzzles are ingeniously constructed if not entirely convincing; the narrative moves smartly along, with a fair measure of tension. The problems here revolve around the characters: they all chat away like ninth graders on a picnic--the aliens are simply teen-agers with some extra eyes, tentacles, legs or whatnots. And for some reason, Clement chooses to end the adventure abruptly, without resolving anything--the feeble postscript just doesn't do the job. Intriguing, sometimes, but frothy and eccentric.