Each year, the ten men went out, shining dark mail about their heads, onto the seaweed lakes of Europa. They called it seaweed, but really that was an Old Earth word, and with it came the traditions and revels of Earth culture, the people lining the streets with boughs of green as the ten men left the biospheres beneath the deep sea and rose upwards in their ten craft towards the surface.

On the other side of the planet, from a crack in the surface, the water was falling - slowly enough that it did not trouble the inhabitants above, but steadily enough that 10,000 litres were lost each year. The biospheres were near the planet's upper face, but the seaweed fields were lower, near the crack and the river that fell endlessly into space, freezing and dispersing throughout the solar system.

The men were carefully chosen. They were strong and clever - Europa's founders had been NASA scientists - but they were also disposable. A woman was never sent, with her gene-spliced womb and infinite genetic potential. But men, yes. Men went to mow. There was even a song, an ancient ditty that echoed in one of the mowers head's as he broke the surface and began the long journey to the seaweed fields, to gather in the year's harvest, the energy that fuelled their existence.

This was his second year. As the long days of the journey passed, he whistled and looked around him at the endless ocean. Then, after a month or so, the ten men met at the edge of the fields, the sunstar shining bright across the glistering ice, the green bio-engineered shoots a glorious pattern on the surface of the world.

There was one new mower among them. He had been chosen later, after old Adam had retired. He was keen, too keen - even careless. They talked across comms, dividing the kilometres between them, tabling squares they would each take. The mower, satisfied, took himself off to start his harvest, nearly the furthest out. There was only one further out. The patch closest to the endless river into space was the new mower's. He'd pushed for it, and, used to harmony, no one had disagreed. It was smaller than the others.

On the third day of mowing, the first mower felt the waters boom and saw the wave. From out of the depths it came, neither man nor machine, a tsunami wave, relentless and double-prowed, rucking the ice and ruining half the harvest before it. The mower's heart raced. He set his engines to full and his craft half-leapt into the wave, although he did not know what that was, instinct driving him home against the current. The force of the machine counteracted the wave, and he held his place. The comms of the others started to bubble in the background of his systems, many half-broken by the wave, some missing. Signals came from around him, shouts and messages, and it seemed the ten men all remained afloat. So they began the journey home early - this was too dangerous, the uneven deeps.

The craft with their small cargo of harvests, culled by their great sweeping arched blades, began the long journey together, each of the men afraid in his own way, wanting the biosphere and looking inwards, thinking of his family and friends and Europa's womblike waters above their heads again.

They did not know, they said later - they did not realise, though some of their navs were still working and could have mapped the seas for them - until they were only a few days away from home. They did not know that the tenth man was missing. The new mower, the young man. He had not been on anybody's screen on the long journey home, and they had not heard the beep of his comm. His voice had been missing. And so they came home, nine men out of the deeps. And the tenth man out there, beneath the planet's waters in his broken craft or out in the icy fountains of the river that falls, on and on, out into space.