Neptune's Cauldron — Michael Coney

Pursued by the interplanetary police for a crime he did not commit, space traveller Tyg is forced down on the planet Storm, where he finds a revolution brewing among the Tadda against King Caiman, the planet's tyrannical ruler.
He must prove his innocence of the crime with which he is charged, as he fights for survival beneath the Storm's seething oceans, where the very existence of the Tadda is threatened by the deadly undersea volcano known as NEPTUNE'S CAULDRON.