(CNN) In early November, an international diving expedition set out from Indonesia into the Sea of Java to find the wrecks of several Dutch warships that sank during World War II.

The team planned to place a plaque next to the wreck of a Dutch admiral's warship to commemorate the upcoming 75th anniversary of its sinking.

Instead, the expedition made a startling discovery.

"We were and are all shocked that the ships were gone," said Lt. Cmdr. Paul Middelberg, a spokesman for the Dutch Ministry of Defense.

The wrecks of the admiral's ship, the cruiser HNMLS De Ruyter and another Dutch cruiser, HNLMS Java, had completely disappeared. Most of the remains of the Dutch destroyer HNLMS Kortenaer were also missing.

According to the Dutch Ministry of Defense, "divers did find tracks and sonar recordings showing the imprints of ships at the bottom of the sea."

And it wasn't only Dutch warships that disappeared.

All traces of at least two British warships that also sank in the Battle of Java Sea apparently had apparently also gone missing, Middelberg said.

The British and Dutch governments issued public condemnations.

The US Navy also released a statement expressing concern "that the wreck of the USS Perch, a sovereign vessel, has been salvaged without the permission or knowledge of the US government."

Photos: Wrecks of WWII warships missing An international team of divers has set out to find the wrecks of several Dutch warships that sank during World War II, including the HNLMS De Ruyter. Hide Caption 1 of 8 Photos: Wrecks of WWII warships missing Cruiser HNLMS Java was sunk during the Battle of the Java Sea on February 27, 1942. Hide Caption 2 of 8 Photos: Wrecks of WWII warships missing HNLMS De Ruyter was also downed that day, killing 235 of 437 crew members. Hide Caption 3 of 8 Photos: Wrecks of WWII warships missing Other navies also lost ships in Indonesia. British heavy cruiser HMS Exeter sank on 1 March 1942 after a battle with Japanese cruisers off Java. Hide Caption 4 of 8 Photos: Wrecks of WWII warships missing The HMAS Perth, a Royal Australian Navy light cruiser was sunk in the Battle of the Sunda Strait on March 1, 1942. Hide Caption 5 of 8 Photos: Wrecks of WWII warships missing The US has worked to retrieve ships lost in the Java Sea, including the USS Houston, sunk in February 1942. Hide Caption 6 of 8 Photos: Wrecks of WWII warships missing Representatives from Australia, the US and Indonesia visited the graves of HMAS Perth and USS Houston on the 73rd anniversary of the Battle of the Sunda Strait. Hide Caption 7 of 8 Photos: Wrecks of WWII warships missing US sailors lower a wreath in the area the ships sank on the 73rd anniversary of the Battle of the Sunda Strait. Hide Caption 8 of 8

Deadly defeat

The USS Perch -- a submarine -- was one of at least eight vessels to be sunk in one of the worst naval defeats the Allies faced during World War II.

The battle began on February 27, 1942, when the Dutch Adm. Karel Doorman led a coalition fleet of American, Dutch, British and Australian warships in an effort to protect Dutch-ruled Indonesia from invasion by Japan.

Throughout several days of clashes, the Japanese decimated the Allied fleet.

Doorman died aboard his flagship after the cruiser was sunk by a Japanese torpedo.

The Japanese fleet also sank the Dutch ships Java and Kortenaer, the British destroyers HMS Jupiter, HMS Electra and cruiser HMS Exeter, as well as the US submarine Perch.

In a battle in the nearby Sunda Strait on March 1, 1942, Japanese ships succeeded in sinking the vastly outnumbered Australian cruiser HMAS Perth and USS Houston, which had been the flagship of the US Asiatic Fleet.

Thousands of sailors died with their ships.

"There were so many fatalities during that early part of World War II, it is just mind-boggling how many sunken ships there are in that area, " said John Schwarz, executive director of the USS Houston Survivors' Association and Next Generations.

In 1942, Schwarz's father Otto was a teenage sailor aboard the Houston who barely survived its sinking.

"He literally was the only person who got out of the lower deck turret number one team, because he was a young man only 17-years-old," Schwarz said.

"Any untold hundreds of (sailors) literally went down with the ship including many who may have been sealed off in sealed compartments," he added.

US sailors have worked to recover ships lost in the Java Sea, including the USS Houston.

Undersea looting

The shipwrecks had all been located within the waters of Indonesia's exclusive economic zone.

Senior Indonesian officials appeared baffled by their disappearance.

"It hasn't been identified whether it has moved or it has been stolen," Indonesian Foreign Ministry spokesman Arrmanatha Nasir told CNN.

"The point is it is not there, where it once was there," he added, suggesting the wrecks may have somehow "shifted" on their own underwater.

Photos: Discovered: Incredible treasures found in unexpected places The earliest known stone copy of the Ten Commandments sold at auction in Beverly Hills in 2016 for $850,000. Hide Caption 1 of 16 Photos: Discovered: Incredible treasures found in unexpected places The stone was first uncovered in 1913. Hide Caption 2 of 16 Photos: Discovered: Incredible treasures found in unexpected places "The tablet's significance is testament to the deep roots and enduring power of the Commandments that still form the basis of three of the world's great religions: Judaism, Christianity and Islam," says David Michaels, director of ancient coins for Heritage Auctions , who will be conducting the sale. Hide Caption 3 of 16 Photos: Discovered: Incredible treasures found in unexpected places Specialists at Mossgreen auctions in Australia discovered this Ming dynasty banknote hidden inside the head of this 14th century Buddhist carving. The wooden sculpture represents the head of a Luohan -- an enlightened person who has reached Nirvana in Buddhist culture. Hide Caption 4 of 16 Photos: Discovered: Incredible treasures found in unexpected places Its face value was worth roughly $98 at the time of its circulation and the 700-year-old banknote is believed to have been handmade during China's Ming dynasty. Together, the banknote and sculpture are expected to fetch between $30,000 to $45,000 at auction. Hide Caption 5 of 16 Photos: Discovered: Incredible treasures found in unexpected places After studying the banknote and carving details, art specialists were able to estimate the sculpture's age, which dates back to China's Hongwu period in the 14th century. Hide Caption 6 of 16 Photos: Discovered: Incredible treasures found in unexpected places Mossgreen specialists believe this is the first time a Ming dynasty banknote has been found inside a wooden Buddhist sculpture. They say it's more common to find relics and semi-precious stones left by monks inside Buddhist sculptures. Hide Caption 7 of 16 Photos: Discovered: Incredible treasures found in unexpected places Ten ancient Roman and Ottoman coins were recently discovered in castle ruins in Okinawa, Japan . This image shows the front of one of the Roman coins. Hide Caption 8 of 16 Photos: Discovered: Incredible treasures found in unexpected places "I couldn't believe they'd found coins from the Roman empire in Kasturen castle," archiologist Hiroyuki Miyagi, from Okinawa International University, told CNN. This is the front of one of the Ottoman coins. Hide Caption 9 of 16 Photos: Discovered: Incredible treasures found in unexpected places Archaeologists recently unearthed a rare 2,000 year old Roman coin during a scientific dig in Jerusalem. The gold coin features the face of Emperor Nero and was likely struck in 56-57 AD. Hide Caption 10 of 16 Photos: Discovered: Incredible treasures found in unexpected places In August 2016, the world's largest pearl was discovered under a bed in the Philippines, where it had lain forgotten for over ten years. Hide Caption 11 of 16 Photos: Discovered: Incredible treasures found in unexpected places This 1,109 carat, tennis ball-sized diamond made headlines in November 2015 when it was pulled out of the Karowe Mine, in Botswana, by Canadian company Lucara Diamond Corp. Hide Caption 12 of 16 Photos: Discovered: Incredible treasures found in unexpected places In May 2016, divers discovered a 1,600-year-old shipwreck while swimming the ancient Roman port of Caesarea. These rare bronze artifacts were recovered from the ship. Hide Caption 13 of 16 Photos: Discovered: Incredible treasures found in unexpected places Over 1,300 pounds (590 kg) of bronze Roman coins dating to the 3rd century A.D. were unearthed in April 2016 by construction workers digging a trench in Spain. Hide Caption 14 of 16 Photos: Discovered: Incredible treasures found in unexpected places In January 2016, over 4,000 Roman coins were discovered by a fruit and vegetable farmer on a molehill in his cherry orchard in Switzerland. Hide Caption 15 of 16 Photos: Discovered: Incredible treasures found in unexpected places The bead on this bracelet, from Portland-based design company William Henry, is actually made using fossilized mammoth tooth and dinosaur bones. Hide Caption 16 of 16

The Indonesian government has agreed to work with Dutch authorities on a joint investigation into the matter.

"The problem in southeast Asia is that there's a lot of looting," said Mark Staniforth, a marine archaeologist with Flinders University in Australia.

"Somebody has been salvaging metal from naval vessels in the region of Indonesia for years," he added.

In 2014, the US Navy discovered "systematic and ongoing disturbance" of the wreck of the Houston during a diving expedition.

Looters had ripped out the majority of the Houston's portholes, removed rivets holding the ship's hull together, and had been in the process of gathering unexploded shells and ordnance.

Valuable scrap metal

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Academics and workers in the Indonesian ship salvage industry tell CNN that the precious and scrap metals that can be scavenged from a shipwreck can be worth hundreds of thousands of dollars.

"The bronze propellers off of larger vessels are worth 40 to 50 thousands dollars," said Professor Staniforth.

According to the Dutch-led diving expedition, in addition to the missing ships, the propellers from the wreck of the British destroyer Electra had been removed.

Shipbreakers on the Indonesian island of Madura who make a living harvesting metal from retired ships explained in detail how a wreck could be lifted from the bottom of the Java Sea.

"You need compressors to breath underwater using tubes. To cut the metal, you need an underwater electric weld. After cutting the pieces, you use a crane to haul them up to the surface," said Mulyadi, a crew manager at a junk shipyard in Madura.

But he said the cost of pulling an entire shipwreck from estimated depths of 60 meters below sea level would be prohibitive.

"The problem is the waters are too deep," he said.

While the US Navy says it is working with the Indonesian government to protect the Houston from further looting, news of the possible scavenging of entire World War II shipwrecks have devastated some veterans' organizations.

"We are on egg-shells," said John Schwarz of the Houston Survivors' Association.

"We're just praying and hoping that no further damage gets done to either our ship or any others."

Undersea war grave

The original USS Houston CA-30 ship's bell was recovered by divers in the 1970s. It now sits atop the USS Houston CA-30 monument in downtown Houston, Texas.

"Wrecks of warships are the final resting place of killed soldiers and are therefore war graves," said the Dutch Ministry of Defense in a statement last month.

"Desecration is a serious offense," it added.

Veterans' groups say the shipwrecks should be protected much like any other cemetery holding the graves of fallen servicemen and women.

John Schwarz, whose father founded the USS Houston Survivors' Association, argued that scavenging wrecks that are believed to hold the remains of hundreds of sailors would be the same as "going into Arlington National Cemetery with excavating equipment, and digging up coffins and graves."

"It is truly a sacred burial ground," Schwarz said.

He urged the Indonesian government to do its part "to protect the sanctity of those ships."