Ever since he failed to find the reef he thought he saw and the boat he was sure he had glimpsed, the lost swimmer has become conscious of the gulf he hangs over. At least the empty but navigable plain which surrounds him horizontally spreads itself beneath the sun's broad eye. Finding his way home again, back to life, will be a matter of simple luck or simple physics. A puff of wind here, an eddy there, and he will be reunited with his boat. If for a moment he were able to raise himself only fifty feet above the water, he would spot it at once and the entire traumatic incident would be at an end.
Beneath him, though, lies a dimension which absolutely refuses to reduce itself to a matter of simple physics. The seabed is roughly 1,000 meters away - perhaps 1,500 if he is farther from land than he thought. A mile of water, in short. The swimmer tries to remember what a mile looks like. The entire length of Oxford Street, centre Point to Marble Arch, but stood on end. As he contemplates this, something unseen like a gush of sepia roars soundlessly up at him from below, without warning, blotting out the sunlit layer which swathes him. This chill black torrent is overwhelming in its despair. It is as though a microscopic ghost had arisen from every test and skeleton of the uncounted radiolaria and plankton bedded on the bottom and had suddenly joined in a great upward fume. Far, far below, the basalt itself is calling in a language of eons and its empty message echoes up and spreads around him in a freezing, inky pool. This tectonic voice paralyzes him. It mocks all human hope. It is worse than his first panic, worse even than the threat of death.
James Hamilton-Paterson, Nine-Tenths: The Sea and Its Threshholds