Eleven Hundred Atmospheres

In the dry lab below decks, Dr Yara Costa was setting up a demonstration. Polly suspected it 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 small one went to the bottom of the trench last week, in a mesh bag on the outside of the submersible. It came back like this."

Polly tilted her head. The shrunken cup was strange. The pattern of grooves on its surface was still recognisable. But every dimension had shrunk by about two thirds. The walls were now densely packed. It looked like a tiny ceramic.

Yara explained. Styrofoam is about ninety-eight percent gas, trapped in plastic. At full ocean depth, the gas gets pressed out. The plastic crushes in to fill the empty space. The chemistry does not change. Only the volume.

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

"For the pressure hull of the submersible, this does not happen," Yara said. "Because the inside is also at sea-level pressure. The danger is not being crushed. The danger is being forced to leak."

She had a second demonstration. She put a single marshmallow on a clean tray. Then she sealed it inside a glass jar and pumped out all the air.

The marshmallow began 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 let the air back in, the marshmallow shrank back to almost its original shape. But not quite. "This is the opposite direction," Yara said. "We took the pressure off. The marshmallow tried to expand. The trench works the other way. It tries to crush everything."

She showed Polly one last thing. At certain depths and temperatures, methane and other gases stay locked into a crystal made with water. These are called methane hydrates. They are stable only under pressure.

Bring a piece to the surface, and it falls apart, releasing the gas. Yara had a sample in a small freezer. It looked like dirty ice. She did not bring it out.

"At eleven hundred atmospheres," she said, "water is still a liquid. But it is denser than at sea level. About two and a half percent denser. Sound travels faster in it. A falling submersible feels heavier the further down it goes." Polly looked at the shrunken cup. A whole world that did not work the way the world up here worked.