The coral nursery underwater, planted by divers for five years

In a bay in the South China Sea, six meters below the surface, there is a field.

You cannot see it from the boat. The water gives nothing away. Then you drop through the top layer of light and the grid appears below you, frame after frame, each one strung with small dark shapes that look at first like debris.

Iron frames are bolted to the seabed in a grid, and what is tied to them is not seedlings but fragments of coral the size of a fist. The divers call them coral babies. A healthy branch is cut from a mother colony, tied onto an artificial frame, and planted the way rice is planted. If it takes, it grows a few centimeters a year. In five years it is a small thicket.

The people planting are a group of diving enthusiasts. The founder is called Dahai, Big Sea, and he has been diving for twenty years. He watched the saddest change this water has gone through: the coral bleached and degraded, and the bottom went from a bright tropical rainforest to gray rubble. Fewer fish, fewer snails. Even a diver with his hours began to find the seabed boring.

Boring. The word stung him. He read up and found that coral reefs support a quarter of all marine life, and that reefs around the world are bleaching on a vast scale. He gathered his friends from the diving community, went to research institutes for advice, and learned the technique of transplanting fragments. Coral can reproduce asexually. A healthy fragment will regrow and attach itself given the right conditions.

Planting coral is harder than farming. Every descent is an operation: thirty or forty kilos of gear on your back, a frame carried down, then positioning and fixing and tying off in the current, one hand holding station while the other works. Water temperature, light, typhoons, an anchor chain dragging across the site, all of it is a variable. Of the first hundred fragments, sixty percent were alive after three months. The day the number came in, Dahai posted a selfie taken underwater, holding a slate: They're alive.

Five years on, several hundred fragments have grown into small thickets on the frames. The monitoring data shows fish numbers in the nursery area clearly rising; a thicket is a kindergarten for fish. The divers took on a second job as well, the value-added kind: clearing trash from the seabed around the nursery on a schedule, and persuading boats not to anchor where they should not. "We're not only planting coral. We're redecorating a piece of the seabed."

Planting coral is a romance measured in years. The small piece you tie down today is, five years from now, a breathing thicket of color — even though you never see the whole of it flower.

Beside the nursery the research institute mounted an underwater camera and streams the picture live, so anyone in the world can farm coral from a screen. A child left a comment online: "Uncle, when does the coral bloom?" A diver wrote back: "Coral doesn't bloom. But when you're grown up, come down and look for yourself. It's better than flowers, and it's alive."

The seabed farm has an organization behind it now. The nursery works with marine research institutes, and the divers have become a civilian caretaking team: which species to plant, where to plant it, how to monitor it, all of it under scientific guidance. The entry requirements are stated plainly. An advanced diving certificate, and the willingness to follow research protocol. "Everyone has enthusiasm. The seabed doesn't need enthusiasm. It needs discipline."