“How deep?” Greg said, leaning around to stare out into the dark void beyond Jack and Yegor.

“In this cavern, or in the oceans?” Jack asked.

“Both,” Abby said, leaning back.

“Well, in here, our soundings having it drop to below eight thousand feet in certain places – that certainly is deep – crush depths for sure. But it’s nothing compared to what’s in the ocean trenches.” Jack spun in his seat to face the group. “Cate mentioned the different depths we have in the ocean. It’s actually a bit like the Earth’s geology and can be defined in layers – the crust, the mantle, outer core and inner core – so too can the world’s oceans be thought of as existing in layers.”

Jack used his hands, to make layers in the air. He started at the top. “Most everything we know lives in the upper-most layer, called the pelagic zone; that’s above the continental shelf. But below that are several more layers that are largely unexplored.” He grinned. “Did you know that ninety-five per cent of the underwater world remains unexplored? Down in the deep and dark, are three more zones – below three thousand feet, we enter the Bathyal Zone – it’s also called the midnight zone as it’s the first layer totally devoid of light.”

Yegor grunted. “We can dive to this depth in the Prusalka if need be.”

Jack nodded. “But the ocean goes even deeper, dropping down farther to below thirteen thousand feet or so, and into the abyssal zone – the abyss – home of the giant tubeworm and giant squid, and that one goes all the way down to twenty thousand feet.”

Cate leaned forward resting her elbows on her knees. “But there’s one more zone to go – the Hadal – my favorite, and named after the realm of Hades. That one drops down to thirty-six thousand feet, and touches the deepest ocean trenches on Earth. We have no real idea what’s down there. And when our deepest submersibles make it down, they can only spend a few minutes looking at about a dozen square feet of the environment. Imagine dropping into the Amazon jungle for a few minutes, at night and in a coffin, with a flashlight and tiny window to see through.” She took a small sip from her water bottle. “We don’t know what lives down there… if anything even could.”

Jack nodded, holding her gaze and hoping she was thawing to him again. “That’s right, and for that matter, we aren’t even sure what really lives in the dark layers above either. The world still has plenty of mysteries, and the ocean is the prize winner for keeping the most.”

“Well, thank god, we’re in this cave, where it’s nice and safe,” Greg said, grinning.

Another pop and groan from the hull seemed to punctuate Greg’s comment, making everyone look at the steel ceiling above their heads.

“Safe-ish,” Greg added.