Read the following extract from an account by Mara Venn, who joins a scientific expedition in a research submersible.

At 5.40 in the morning, the crane lifted us from the deck. From inside, I could see only a section of cable rising through the pale sky, but a camera showed the whole machine hanging above the ship: a stubby orange capsule, scratched from earlier descents, with its sampling arm folded beneath it. On deck it had seemed solid enough to resist anything. Suspended over the water, it looked like an armoured seed held between two fingers.

The crew guided us outwards with ropes. One of them kept his palm pressed against the hull until the widening gap forced him to let go. Then the sea was directly below us, black and softly folding. The crane released its cable and we dropped the last half-metre with a slap that shook my teeth. Water climbed over the window. The ship, the crew and the morning disappeared together.

The hatch had been closed for only ten minutes, yet the cabin seemed to be shrinking around the three of us. My knees touched a bank of switches. The pilot's shoulder was against mine. As we descended, small sounds travelled through the metal: a tick near my ear, a dry creak beneath my feet, then another tick. The pilot called them 'the machine learning the load'. He spoke casually, but I watched the depth display repeat its steady subtraction of the world above: 180 metres, 240, 310.

At 900 metres, the darkness did not merely surround us; it appeared to possess everything beyond the glass. Then the pilot switched on the lamps. Two white roads opened across the seabed. Fine mud lifted and rolled away. When the sampling arm unfolded, it no longer resembled the helpless limb I had seen beneath the crane. Joint after joint extended with patient certainty. Its metal fingers closed around a piece of rock, turned it once and placed it exactly inside the collection basket. The movement was slow, but nothing in it was hesitant.

A warning sounded as we prepared to take a second sample. It was not a siren, only a single electronic note and an amber message reporting reduced pressure in the arm's hydraulic system. Even so, every person in the cabin became still. The pilot stopped the arm, checked the readings and began folding it back against the hull. The great mechanical hand that had lifted stone so neatly was suddenly dependent on a narrow hose, a seal and the judgement of the person beside me.

We rose without the second sample. For more than an hour, the numbers on the display counted us back towards daylight. When the crane finally returned the submersible to the deck, it landed with a heavy certainty that made the surrounding crew step away. I climbed out and placed my hand on its scarred side. The machine had carried us through a depth that had erased the entire ship from view, yet a damaged line no wider than my thumb had been enough to send it home.