A video of the crew members singing a "goodbye" song inside the KRI Nanggala-402 submarine just weeks prior to its sinking has just made the entire tragedy all the more difficult to come to terms with.

Last week, it was reported by Indonesian officials that they had lost all contact with the KRI Nanggala-402 submarine.

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The vessel was first declared missing on April 21st after it failed to report back with the results of a training exercise while in deep waters in North Bali.

First Admiral, Julius Widjojono, said it had been taking part in a torpedo drill when it became untraceable at approximately 4:30 AM.

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​Military chief Hadi Tjahjanto then confirmed to Reuters that there were a total of fifty-three people on board the vessel at the time it was declared missing.

"We are still searching in the waters of Bali, 60 miles (96 km) from Bali, (for) 53 people," he said.

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Military information group Janes reports that the navy sent a distress call to the International Submarine Escape and Rescue Liaison Office, just before 9.40 AM and that prompted the search to get underway.

Support from Singapore and Australia meant that more ground could be covered, as "several" ships were sent out just 26.5 miles northwest off of Singaraja.

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"The TNI-AL has dispatched several warships to the area, including the hydrographic ship, KRI Rigel (933), for the search," the report from Janes continued. "Other vessels currently on the scene include the TNI-AL's first-of-class Fatahillah corvette (361), the lead Bung Tomo-class corvette (357), and the Kapitan Pattimura (Parchim I) class corvette, Teuku Umar (385)."

However, just 3 days into the search, officials had stated that the vessel's oxygen supply would have run out over the weekend.

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And within that time, the status of the submarine changed from "missing" to "sunken."

​Everyone on board was presumed to be dead.

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But as the search continued to get more frantic, some still clinging onto the hope that it might have just reached untraceable territory, it was confirmed just a day later that the wreckage had been found at the bottom of the sea.

On Sunday, Tjahjanto told reporters this:

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​"We received underwater pictures that are confirmed as the parts of the submarine, including its rear vertical rudder, anchors, outer pressure body, embossed dive rudder, and other ship parts."

"With this authentic evidence, we can declare that KRI Nanggala 402 has sunk and all the crew members are dead," he added.

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Yudo Margono, Navy chief of staff went into the finer details of the tragic accident.

"The KRI Nanggala is divided into three parts, the hull of the ship, the stern of the ship, and the main parts are all separated, with the main part found cracked."

Messages of condolences have been pouring in from all over the world...

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The cause of the disappearance remains uncertain.

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But the navy had previously said an electrical failure could have left the submarine unable to execute emergency procedures to resurface and have also stressed that none of the crew members are responsible for that.

The KRI Nanggala 402 submarine was built in 1981 in Germany and was later revamped in 2012 in South Korea.

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Reuters reported that Indonesia has been "seeking to upgrade its defense capabilities," as some of its equipment currently in service is "old."

Indonesia's President, Joko Widodo, said "all of us Indonesians express our deep sorrow over this tragedy, especially to the families of the submarine crew."

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In order to honor the lives lost, a video was shared by the Indonesian navy showing the crew inside singing just a few weeks prior to the accident.

The most heart-wrenching part is that they're singing a song called "Sampai Jumpa..."

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Which translates as "until we meet."

The crew members look happy as they sing the lyrics of the song.

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Now the video has since gone viral with many leaving more tributes under it.

Watch the full, heartbreaking video here:

RIP to all fifty-three of the lives lost during this tragic event. Our thoughts and prayers are with the families affected.