One of those times a crazy-looking sci-fi paperback cover is actually true to the story. The main characters float past a bunch of huge ruined statues sticking out of the water, left behind by an long-gone advanced civilization. (Ahhhh... but are they REALLY gone?)

There's a long sequence of trudging through alien jungles, with great descriptions of weird alien lifeforms. Black trees covered in glowing molluscs...

Good stuff.

From the author that gave us the movie Fantastic Planet (novel: Oms en serie).