A pair of warships lost during a historic 1942 naval battle have completely disappeared from their resting places at the bottom of the Java Sea. Large portions of a third ship are also missing. An international investigation has been launched in hopes of solving this bizarre maritime mystery.

The Netherlands defense ministry has confirmed that two of its ships lost during the Battle of the Java Sea—the HNLMS de Ruyter and HNLMS Java—have vanished, while a third ship, the HNLMS Kortenaer, appears to be missing some of its parts. The wrecks were rediscovered back in 2002, but a new expedition to mark the 75th anniversary of the historic battle came up short. Sonar images showed imprints of where the wrecks used to be on the ocean floor—but no ships.

“An investigation has been launched to see what has happened to the wrecks, while the cabinet has been informed,” noted the defense ministry in a statement. “The desecration of a war grave is a serious offense,” hinting that the wrecks were illegally salvaged. //

This practice is in contravention of laws set up to protect these historically sensitive sites. Around 2,200 people died when these ships went down, and the wrecks have been declared sacred war graves. “The people who died there should be left in peace,” said Theo Vleugels, director of the Dutch War Graves Foundation, in The Guardian.