More than 100 World War II-era shipwrecks decorate the seafloor around Indonesia, Malaysia and Singapore — and now, there are two fewer.

According to Dutch media reports, a pair of submarines that sank off the coast of Malaysia in 1941 mysteriously vanished late last week, leaving behind only some broken scraps and ghostly outlines in the sand. The wrecked subs — Dutch vessels named HNLMS O 16 and HNLMS K XVII — also contained the remains of 79 crewmen, which are now missing. [17 Mysterious Shipwrecks You Can See on Google Earth]

How does a shipwreck simply disappear? According to Dutch government officials, the subs were likely stolen by scrap-metal scavengers, who have made a habit of pilfering old wrecks from the region. As many as 40 World War II-era ships have been partially or completely dismantled by scavengers, a 2017 report by the Guardian found, resulting in the desecration of the remains of some 4,500 crewmen who went down with their ships.

Salvaging a shipwreck usually requires blowing the vessel apart with explosives, then spending days or weeks hauling any valuable metals up onto the surface with a crane. For their trouble, scavengers can come away with millions of dollars' worth of steel per ransacked ship, plus other spoils, such as copper cables and phosphor bronze propellers, according to the Guardian article.

Wartime shipwrecks are protected under international treaties as the unmarked graves of departed soldiers — however, that has not stopped salvagers from destroying the wrecks of the American, British, Dutch, English, Australian and Japanese vessels resting in South East Asian waters.

In March 2018, Malaysian officials signed an agreement with the Dutch foreign minister to better protect Dutch war wrecks in Malaysia’s waters. (Parts of Malaysia were once under Dutch colonial rule.) The agreement followed a string of particularly serious shipwreck desecrations; in 2016, the wrecks of three Dutch warships vanished from the bottom of the Java Sea off the coast of Indonesia, along with the remains of 2,200 people, the Guardian reported.

Originally published on Live Science.