Oahu, the third largest of the Hawaiian Islands and the home of Honolulu, is dissolving from the inside, shows a new study. Geologists state that the island’s Koolau and Waianae mountains will gradually be reduced to nothing, leaving Oahu a flat, low-lying island.

“We tried to figure out how fast the island is going away and what the influence of climate is on that rate,” said Steve Nelson, a Brigham Young University geologist. “More material is dissolving from those islands than what is being carried off through erosion.”

The study, published in Geochimica et Cosmochimica Acta, looked at both groundwater and stream water, comparing them to see which removes more mineral material. Geologists spent two months sampling both types on Oahu, then added data from the U.S. Geological Survey to calculate the total mass that disappeared from the island in various years.

“All of the Hawaiian Islands are made of just one kind of rock,” said Nelson. “The weathering rates are variable, too, because rainfall is so variable, so it’s a great natural laboratory.”

According to Nelson and his colleagues’ estimates, the plate tectonics affecting Oahu will continue to grow the island for 1.5 million years. After that, groundwater will begin to force the island and its mountains into a flat landscape.