Eleven Hundred Atmospheres

In the dry lab below decks, Dr Yara Costa was preparing a demonstration that Polly suspected was for her benefit. There were two Styrofoam cups on the bench. One was full-sized. The other was about the size of a thimble.

"Same cup," Yara said. "Originally. The smaller one went to the bottom of the trench in a mesh bag on the outside of the submersible last week. It came back like this."

Polly tilted her green head. The shrunken cup was an extreme thing. The pattern of grooves on its surface was still recognisable, it had once been the same shape, but every dimension was reduced by perhaps two-thirds. The walls, formerly thin foam, were now densely packed. It looked like a tiny ceramic.

This, Yara explained, was what 1,100 atmospheres of pressure did to anything with air in it. Styrofoam is about ninety-eight percent gas trapped in plastic. At full ocean depth the gas was pressed out of the foam and the plastic crushed in to fill the void. The chemistry of the plastic didn't change. Only its volume.

Polly hopped along the bench, examining the cup from every angle.

"For the pressure hull, it doesn't compress," Yara said. "Because the inside is also at the pressure of the inside. The danger isn't being crushed. The danger is being slowly forced to leak."

She had a second demonstration. She produced a small steel container with a tightly fitting lid. Inside was a roll of fresh marshmallows, the kind kids toast over campfires. She put one on a clean tray inside a clear bell jar. Then she sealed the jar and pumped the air out of it with a vacuum pump connected to a hose.

The marshmallow on the tray began, slowly, to swell. Within thirty seconds it had doubled in size. After a minute it was three times its original volume. Polly's eyes went wide. When Yara released the vacuum, the marshmallow shrank back almost to its original shape, but not quite.

"That's the opposite direction," Yara said. "We took the pressure off the marshmallow. It tried to expand. The trench works in reverse, it tries to crush everything." She tossed the deflated marshmallow into a bin.

She showed Polly one last thing. At certain depths and temperatures in the trench, methane and other gases that would normally bubble out of solution stay locked into a crystal lattice with water. These are called methane hydrates, and they are stable only under pressure. Bring a piece to the surface and it dissolves rapidly, releasing the trapped gas. Yara had a sample in a small freezer, looking like dirty ice. She did not bring it out.

"At eleven hundred atmospheres," she said, "water is still a liquid. But it's a slightly denser liquid than it is up here. About two and a half percent denser. Sound travels faster in it. A submersible falling feels heavier the further down it goes."

Polly looked at the shrunken Styrofoam cup. A whole world that didn't behave the way the world up here behaved.