Madsen made headlines when he successfully financed the building of the submarine through crowdfunding, completing it in 2008.

He appeared on Danish television on Friday to discuss the submarine’s sinking and his rescue. Footage aired on Denmark’s TV2 channel showed him getting off what appeared to be a private boat and making a thumbs-up sign as he walked away. “I am fine, but sad because Nautilus went down,” he told TV2.

Madsen said “a minor problem with a ballast tank … turned into a major issue” that ultimately caused the sinking of the vessel, which was considered to be the largest privately built submarine of its kind. The ballast tank is a compartment that holds water to provide stability for a vessel.

“It took about 30 seconds for Nautilus to sink, and I couldn’t close any hatches or anything,” Madsen said. “But I guess that was pretty good because I otherwise still would have been down there.”

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Authorities were alerted to issues with the voyage when Wall’s boyfriend reported her missing early on Friday. Two helicopters and three ships searched the sea from Copenhagen to the Baltic Sea island of Bornholm.

The navy initially said the craft was “found sailing” south of Copenhagen. But Damgaard later said the 40 tonne submarine had sunk.

Kristian Isbak, who had responded to the navy’s call to help locate the ship, sailed out immediately Friday and saw Madsen standing wearing his trademark military fatigues in the submarine’s tower while it was still afloat.

“He then climbed down inside the submarine and there was then some kind of air flow coming up and the submarine started to sink,” Isbak said. “[He] came up again and stayed in the tower until water came into it” before swimming to a nearby boat as the submarine sank, he added.