(NewsNation) — Crews are searching for a missing submarine on a mission to view the wreck of the Titanic. The Titan lost contact with the crew of Canadian research vessel Polar Prince approximately 1 hour and 45 minutes after submerging on Sunday morning.

It’s unclear why the sub lost contact or, if found, what state the vessel and crew may be in. Over the years, few submarine rescues have been completed. Here’s how submarine rescues in the past have happened.

HMS K13, 1917

During the first World War, the Royal Navy HMS K13 was on a training mission in Gareloch, Scotland, with 80 people on board. Once in the lake, the vessel failed to level out at 20 feet, and it was pulled underwater.

Four ventilators in the engine had not been properly closed, flooding the submarine. It was the middle of the night when people on the surface realized something had gone wrong and the rescue mission wasn’t able to be launched until the following day.

Rescuers were able to attach an air line to the ballasts and inflate them to bring the submarine back to the surface where the hull could be cut open in an attempt to rescue the crew. While 48 made it, 32 of the crew did not survive the rescue.

USS Squalus, 1939

In another training incident, the USS Squalus was off the coast of New Hampshire, carrying 56 Navy personnel and three civilians when an air valve in the engine room failed, flooding the compartment. Of those on board, 26 immediately perished as the sub sank under 240 feet of water.

The 32 crew and one civilian in the forward compartments sent up a location marker and used Morse code to communicate with rescuers from the USS Sculpin. In addition to being trapped in a flooded submarine, those on board also faced the risk of chlorine gas that had begun leaking from the battery compartment.

A newly-developed rescue chamber was used to bring the survivors to the surface in four trips. The Navy divers involved in the rescue were awarded the Medal of Honor.

Pisces III, 1973

Two men were aboard the commercial submarine Pisces III as it worked to lay transatlantic telephone cables. As the Canadian submarine was being towed to a ship, the hatch broke off the aft compartment and the vessel plunged to a depth of 1,575 feet.

The men on board were in contact with rescuers as they worked to conserve the 66 hours of oxygen they had left. They positioned themselves as high up as possible and avoided speaking or moving to conserve air.

Over two days, multiple efforts to lift the sub failed. Eventually, crews were able to bring the vessel to the surface but it took another 30 minutes to remove the hatch. When the two crewmen were finally free, they had only 12 minutes of oxygen left.

AS-28, 2005

The Russian AS-28 ran into trouble after becoming tangled in a fishing net during a military exercise. The rescue vessel had five days of oxygen on board, but a Russian Navy spokesperson later said there was only 24 hours of air on board, with no clear reason for the discrepancy.

Initial efforts to rescue the sub failed and Russia called for international help, something they had failed to do in the 2000 sinking of the Kursk nuclear submarine, resulting in the death of all 118 on board.

British and American rescue crews eventually freed the vessel and brought it to the surface with four to six hours of air remaining for the seven crew members on board.