This is a direct sequel to the author's previous book, Fragment, which was one of the best books I read in 2010. That was about an island, the last remaining part of an ancient continent, where evolution went on a very different path. All the animals are vicious and extremely invasive, so if any of them made it to the rest of the planet they'd quickly take over. At the end of the first book everything on this ancient island is nuked except for a few members of an intelligent species. In Pandemonium, two of the scientists who studied Hender's Island are recruited by a Russian billionaire to study an underground ecosystem, which has some very dangerous creatures, which are descended from mollusks. He is in the midst of finishing a self-sustained underground that was partly built during the reign of Stalin, but he's managed to obtain a few surviving creatures from that dreaded island. And they escape, as if the original creatures in these caverns weren't dangerous enough to deal with. It's a race against time for the scientists to escape before this place is nuked as well.
This is a good read, though not nearly as entertaining and harrowing as Fragment.