Indonesian navy ships were scouring the waters off Bali Island on Friday as they raced against time to find a submarine that disappeared two days ago and has less than a day’s supply of oxygen left for its 53 crew, AP reported.

The KRI Nanggala 402 went missing after its last reported dive Wednesday off the resort island, and concern is mounting the submarine may have sunk too deep to reach or recover. The navy chief said the submarine was expected to run out of oxygen by around 3 a.m. Saturday.

There’s been no conclusive evidence the oil slick was from the sub. Navy Chief of Staff Adm. Yudo Margono said oil could have spilled from a crack in the submarine’s fuel tank or the crew could have released fuel and fluids to reduce the vessel’s weight so it could surface.

Margono said an unidentified object with high magnetism was located at a depth of 50 to 100 meters, and officials held out hope it’s the submarine.

The navy also said it believes the submarine sank to a depth of 600-700 meters, much deeper than its collapse depth at which water pressure would be greater than the hull could withstand.