AUSTIN, Tex. — The dormant oil platform known as High Island 389-A rises out of the Gulf of Mexico about 100 miles southeast of Galveston. Below the surface, corals, sea fans and sponges cover its maze of pipes. Schools of jack and snapper, solitary grouper and barracuda circle in its shadows. Dive boats periodically stop at the enormous structure, where dolphins, sea turtles and sharks are often spotted.

Now, 30 years after it was built and months after it was abandoned, it is set to be demolished under Interior Department rules governing nonproducing ocean structures. And when it goes, the lush ecosystem that has grown around it will also vanish. There are now about 650 such oil and gas industry relics, known as idle iron, that may meet this fate.

The federal government estimates that the blasts needed to remove one platform kill 800 fish, although others who have observed the process put the number in the thousands. Much of the marine life on or around the structure dies, either from the explosions to separate the platform from its supports or when it is toppled or towed to shore and recycled as scrap metal.

The prospect of losing so much life has brought together an unusual collection of allies hoping to convert High Island and many similar structures into protected reefs. “These structures attract marine life that normally wouldn’t use the area,” said Greg Stuntz, chairman of ocean and fisheries health at the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University Corpus Christi. “Much is growing on them, from corals up to marine mammals.”