It appears to be a giant cairn, rocks piled on top of each other. Structures like this are known from elsewhere in the world and are sometimes used to mark burials. Researchers do not know if the newly discovered structure was used for this purpose.
"Close inspection by scuba diving revealed that the structure is made of basalt boulders up to 1 m (3.2 feet) long with no apparent construction pattern," the researchers write in their journal article. "The boulders have natural faces with no signs of cutting or chiselling. Similarly, we did not find any sign of arrangement or walls that delineate this structure."