Coral reefs around the world are disappearing. In many places, more than 90% of corals have bleached or died. But an effort is under way to re-grow corals by giving them a bit of electro-shock therapy.
By Clark Boyd
Marine biologist Tom Goreau knows coral. He has been diving among the reefs since before he could walk.
It may be too late to save many of the corals
As the director of the Global Coral Reef Alliance, he is passionate about how extraordinary corals are.
"They're very simple animals. They're an animal that's basically a gut with a ring of tentacles around it," he said.
"And with their tentacles they catch zooplankton, little shrimp and animals swimming in the sea. And they eat animals, they don't plants. And they can't move, so that confuses people.
"We're used to thinking of animals as things that run and swim around. These things live in a limestone cup, and they kind of pull into that cup for protection."
Corals are the only animals in the ocean that build permanent solid structures. It means that they cannot run away from diseases or pollution.
The problem is that they are especially sensitive to both.
"The reason they're so sensitive is that they're not just animals, they are also plants," said Dr Goreau.
"They're plants because in the cells of the coral they have symbiotic algae living inside and those things photosynthesise.
"Corals have to have clean water and a lot of light to live, because the algae are helping the corals grow their skeleton and grow faster and provide a little bit of their food."
The algae are also what give corals their fantastic colour.
But the algae in the coral tissue will die if the coral encounters stress. The coral then bleaches; it turns white or transparent. The coral itself will die if something is not done.
The news since the 1980s is grim. It seems that warmer ocean temperatures are killing off corals at a record rate. Corals that have survived for 1,000 years are dying.
It is happening just about everywhere, but the corals in the Indian Ocean and the Pacific Ocean are especially hard hit.
"The Maldives and Seychelles and Palau and places I've worked - the mortality in many places was close to 99%," said Dr Goreau.
"In some places it only 90 or 95%, but I've been in reefs where you could find only one live coral afterwards.
"In 2002, there was mass mortality all across the South Pacific, and it never gained the attention of governments. In 1998, most of the corals in the Indian Ocean died. So we're right on the edge."
But Dr Goreau and a German architecture professor named Wolf Hilbertz are working to bring coral populations back.
Professor Hilbertz has developed something he calls sea-creation, which he explains in a video produced by Natural History New Zealand.
"Corals are using calcium carbonate drawn from seawater to build their exo-skeletons. We use these materials for building purposes," said Mr Hilbertz.
Marine biologist Tom Goreau is a veteran diver
The key to the process is electricity.
Professor Hilbertz found he could mimic the natural process that corals use to grow their skeletons.
He did it by putting a low voltage current through seawater. The current draws out the minerals, which essentially constitute limestone. But, you have got to put the limestone somewhere.
So Professor Hilbertz has designed what he calls coral arks. They are made of welded steel bars.
He sinks the arks to the sea floor, and then supplies a current. Within a short period of time, limestone will start to grow on the steel.
Professor Hilbertz says the only special equipment needed is a special titanium mesh that can withstand electric current and sea water.
Safe for divers
The limestone that grows on the steel is stronger than concrete. Live corals can then be grafted on to the structure.
Those corals can survive pollution and high sea temperatures, as long as the electricity stays on. The current takes care of growing a coral's skeleton. That frees the animal up to fight off diseases or other stresses.
"What we found was that we were able to grow corals at three to five times the record rates, in a habitat where all the corals had been killed by pollution," said Tom Goreau.
"And we were able to do that as long as we kept the power on. It's the electricity itself that gives them that growth and that extra resistance to stress."
Tom Goreau and Wolf Hilbertz have about 15 coral ark projects going around the world.
They are all pilot projects as they do not have the money to try it yet on a larger scale.
One of the biggest trials is being carried out in the north of the Indonesian island of Bali. There are currently 21 coral arks in a bay there, powered by a bank of 80 chargers on shore.
The electricity costs are equivalent to running a few beach lights. It is only 12 volts so divers are safe.
It seems to be working, with the corals growing quickly.
The team hope that the Bali project can spur interest in their method. However, both men worry that it may be too late to save the majority of corals worldwide.
All photos courtesy of the Global Coral Reef Alliance
Clark Boyd is technology correspondent for The World, a BBC World Service and WGBH-Boston co-production