The 44-year-old submarine, known as KRI Nanggala-402, was last seen early Wednesday as it started a torpedo drill, a navy spokesperson said. The vessel was given permission to dive but it never came back up to share the results of the drill.
Searchers found an oil slick near the spot where the submarine went down, but they did not find the missing vessel after several hours of searching.
“We know the area but it’s quite deep,” First Admiral Julius Widjojono told the AFP.
The submarine is built to withstand pressure at a maximum depth of 250 metres, but officials say the vessel might have gone farther down.
“It is possible that during static diving, a blackout occurred so control was lost and emergency procedures cannot be carried out and the ship falls to a depth of 600-700 metres,” the Indonesian navy said in a statement.
The navy says the oil spill might have been a sign of damage to the fuel tank or a deliberate signal from the missing crew.
“We are still searching in the waters of Bali, 60 miles (96 km) from Bali, (for) 53 people,” military chief Hadi Tjahjanto told Reuters in a text message. He said contact with the vessel was lost at 4:30 a.m. on Wednesday.
The Indonesian navy dispatched two vessels to search the water with sonars. Australia, India and Singapore have also agreed to join the search.
The KRI Nanggala-402 weighs 1,395 tonnes and was originally built in Germany in 1977, then added to Indonesia’s fleet in 1981. The vessel was last retrofitted in South Korea in 2012, the Indonesian Defence Ministry says.
It’s one of five submarines in the Indonesian fleet.
This is the first time Indonesia has lost a submarine, but other nations have lost some in years past.
In 2017, for example, Argentina lost a submarine in the southern Atlantic with 44 crew aboard. The imploded wreck was found a year later.
—With files from Reuters