The underwater object that an American nuclear-powered submarine hit last month was an uncharted underwater mountain, according to a US Navy investigation.

The USS Connecticut, a Seawolf-class submarine, was operating in the South China Sea when it struck an unidentified object on Oct. 2, injuring some crew members and forcing the vessel to head to Guam for repairs.

A probe “determined that Connecticut grounded on an uncharted seamount while operating in international waters in the Indo-Pacific region,” the US Navy said in a statement.

Seamounts are underwater mountains generally formed by volcanic activity that can rise as much as 4,000 meters high from the seafloor.

The investigation’s findings have been submitted to Vice Adm. Karl Thomas, commander of the 7th Fleet, which operates in the Western Pacific and Indian oceans.

He will decide “whether follow-on actions – including accountability – are appropriate,” the statement added.