Small port with limited berth space and two loading stations. (CSIS Asia Maritime Transparency Initiative /DigitalGlobe)

Marine biologist John McManus, who has been studying Pacific coral reefs for the past 30 years, remembers a two-day boat journey a few years ago to a remote part of the Spratly Islands, a chain of low-lying coral and rocky reefs in the South China Sea.

“You are traveling along in open ocean waters, then you come upon a place where the waves are breaking, then everything beyond the reef is flat, like a giant pool,” said McManus, who is director of the National Center for Coral Reef Research at the University of Miami.

Today, seven such coral reefs are being turned into islands, with harbors and landing strips, by the Chinese military. Not only is this work threatening China’s relations with the United States and several other Pacific nations, it is also destroying a rich ecological network, according to McManus.

“This is devastating,” he said. “It’s the worst thing that has happened to coral reefs in our lifetime.” U.S. officials estimate that the Chinese military has built up the shallow tropical seafloor with reclaimed sand, steel, wood and concrete barriers to create 2,000 acres of new territory.

Photos: New satellite imagery of Chinese island-building

Chinese officials have said that “rigorous” testing is done before any construction to protect the reef environment and that beyond military uses the created islands will improve China’s capabilities for “search and rescue at sea, fishing security, disaster prevention and relief, and meteorological monitoring.”

While China’s construction has raised tensions in the region, it is also provoking questions about how long these bases will be able to withstand the severe storms that are frequent in that part of the Pacific Ocean.

“You can build an island if you do it right,” said Robert Dalrymple, a professor of civil engineering at Johns Hopkins University. “But it is not clear these islands will be permanent unless they can deal with erosion. They will wash away, just like putting sand on East Coast beaches.”

Artificial islands have been built for coastal resorts or airports in shallow waters off Florida, the Caribbean, the Arabian Sea and many other areas. The Philippines and Vietnam have erected outposts — many of them on wooden stilts — in the South China Sea in the past two decades in an effort to support territorial claims. But the Chinese efforts dwarf these projects. At some of the new islands, the Chinese are building concrete breakwaters hundreds of feet long to hold the sand in place.

Dalrymple has visited construction projects in China and says the country clearly has the engineering expertise to handle huge amounts of dredged material. Other experts say that the Chinese are working quickly, rather than carefully, to create the artificial islands.

“The engineering feats are incredible in terms of speed,” said Patrick Cronin, a senior analyst at the Center for a New American Security, a Washington think tank. Cronin has been briefed on the island-building by senior U.S. officials. “They have not only doubled the land mass . . . but [also] have created forward staging bases of both military and civilian use. Dredging machines didn’t just build islands, but also dug deeper shipping channels.”

The results can been seen in unclassified satellite images posted by the Asian Maritime Transparency Initiative at the Center for Strategic and International Studies, another think tank. Time-lapse images show coral rings in an azure sea being filled in by white sand dredged from the nearby seafloor, followed by the arrival of construction cranes, workers and then multistory buildings.

Construction of the air base continues with ongoing paving and marking of the airstrip, an added apron, construction of a sensor array and development of additional support facilities. A naval vessel is moored in the port. (CSIS Asia Maritime Transparency Initiative/ DigitalGlobe)

Among the projects described on the CSIS site are:

● An airstrip almost two miles long on Fiery Cross Reef.

● Radar facilities and a helipad on Cuarteron Reef.

● A new dock and gun emplacements on Gaven Reef.

● Expansion on Hughes Reef from an outpost on stilts measuring less than a tenth of an acre to a 380-acre multilevel facility and harbor for both civilian and military ships.

● Dredging at Mischief Reef, which lies within what the Philippines considers its economic zone with floating naval docking stations.

● New piers, access channel and possible landing strip at Subi Reef.

● Desalination pumps and a concrete plant on Johnson South Reef.

While the artificial islands seem solid in the satellite images, the Pacific Ocean isn’t always peaceful, says Steve Elgar, a senior scientist in ocean physics and engineering at the Woods Hole Oceanographic Institute in Woods Hole, Mass.

He wonders how long they will survive the wind-driven waves, some as high as 30 feet, that develop far out at sea and then roll in with no landmass to stop them. Such rocky islands as Hawaii, Guam and the Philippines are surrounded by coral reefs that help break up the force of waves traveling across the ocean. But the new bases in the Spratlys don’t have that protection.

“They are in the middle of the ocean, 1,000 miles from north to the south,” Elgar said. “With these huge fetches [distances that wind blows unobstructed over water], they have got to get big waves just from the wind blowing. They are exposed out in the middle of nowhere.”

Other ocean scientists worry about the effect of dredging and island creation on surrounding marine life. The Spratlys contain major fishing grounds for several Asian nations, and the local marine biodiversity has been on the decline for the past two decades, according to a 2013 study by Australian and Chinese scientists.

The report, which appeared in the journal Conservation Biology, found that coral cover had declined to about 20 percent (from about 60 percent) within the Spratly archipelago over the preceding 10 to 15 years. “Climate change has affected these reefs far less than coastal development, pollution, overfishing, and destructive fishing practices,” according to the report, which warned that the declines in the reefs were “unfolding as China’s research and reef-management capacity are rapidly expanding.”

Greg Mitchell, a professor of marine ecology at the Scripps Institution of Oceanography in La Jolla, Calif., researches Pacific reef ecosystems. He says the new Chinese dredging and installation of concrete piers are probably destroying what’s left of the local ecology.

“If the islands had been left alone, they would probably be very diverse,” Mitchell said. “But all of the fishing fleets from Asia have been there hunting everything from sea cucumbers and giant clams and sharks for fins. My guess is the biodiversity has been altered already. But now, they are burying the ecosystem and destroying it.”

Niiler is a freelance writer based in Maryland.