SpaceX and the U.S. Air Force plan to test hypersonic cargo rockets on Johnston Atoll, a Pacific wildlife refuge home to over ...

The top of the island is now underwater, though just under the surface. Sand, silt, and rubble that originated in the coral reef now fill the lagoon. The coral reef is now considered an atoll.

"These atoll islands have the most threatened fresh groundwater on Earth, and are relied upon by some of the most remote communities," says Flinders University's Professor Adrian Werner.