Cate stayed just behind him. “And we’re still at mid water – let’s take it down and have a look at the bottom.”

Jack nodded and turned to Yegor. “Down fifty feet, Mr Gryzlov.”

“Da… yes, sir.” Yegor pushed the u-shaped wheel forward a half-inch and Prusalka angled down a few degrees, and began to plow through the forest of jellyfish. The huge bells bounced soundlessly off the curved window.

“Anyone know what type they are?” Abby asked.

Cate’s eyes remained firmly fixed on the window. “No, but Jack might have a clue; he’s our expert on all things underwater.”

“I can recognize most,” Jack said. “Growing up I had a voracious appetite for all things to do with sea life – past and present. Aquatic paleontology was my first real hobby.” He stared. “But no idea on the jellyfish – not modern, and as they’re so soft, they never really left any fossil record trace behind. As for the seashells, I’d say nauteloids – those were around for hundreds of millions of years, and still around today – just not at that size.”

“Nautiloidea is an ancient class of cephalopods – related to squid and the octopus,” Mironov said evenly. “Most died out prior to the Ordovician period.”

“Cephalopods? Great, hate to be stuck in a dark cave, underwater, with anything with too many arms,” Greg said.

“You and me both,” said Cate. “I’ll tell you about a nightmare I had once after reading a book about being trapped under the dark Antarctic ice.”

Jack half turned. “The giant man-eating squid thing again?”

“Great,” Abby said.

Cate squeezed Jack’s shoulder, hard. “Don’t worry, Abby, even I know that nothing like that exists in the fossil record.”

“Leveling off at ten feet above sea floor… I mean cave floor,” Yegor said.

“Fantastic,” Cate breathed. “And it is a sea; just one no one knows about.”

Outside the underground sea floor was a plain of grasses, with huge fans waving their gossamer wings, and the odd bulbous sponge poking up like brown cabbages. In amongst the plant life, creatures that resembled tiny many-legged skateboards were busily darting about.

“Ha, trilobites,” Abby pronounced. “One for me.”

“Yes. They were our world’s first real survivors, existing for nearly three hundred million years. Some were the size of your thumb, and others, like these guys, as big as a loaf of bread.” Jack smiled. “My first fossil was a middle Cambrian trilobite – four hundred and ninety million years old.”

A cloud of darker fluid puffed up from out to their side, drawing their attention. One of the trilobites was caught in the claws of a foot-long creature that looked like a long flat lobster.