Dutch daredevil Bob Thissen says his unauthorised exploration of a disused submarine near Lisbon was one of his best-ever adventures.
Thissen has travelled to 55 countries over the last decade or so, quenching his appetite for abandoned relics.
But the 32-year-old animator, from Heerlen, says his submarine stunt in Portugal ranks among his top 10 experiences.
He boarded the vessel — docked near Lisbon and thought to have sat unused since 2005 — in April but only published a video of his adventure last week.
“The inside was amazing, everything was still in place, all the documents, all the gauges, radars, it was really cool,” Thissen told Euronews.
“There was also all the stuff from the navy as well: games and things like that which gave an insight into their life on the submarine.”
The adventurer was tipped off by a friend about the existence of the submarine and travelled to Lisbon with his friend Frederik, from Belgium.
Their first attempt to board was thwarted by low tide, but conditions were better when they went back the next day.
Thissen said they swam to the submarine and squeezed through a small hole without breaking anything.
“I’ve been to thousands of locations in more than 55 countries on four continents and I guess it was in the top 10,” he said.
“It was a really great experience and a very unique location. You don’t see abandoned submarines every day!
“Normally they are in an active military base and there’s no way you can get inside, but this one was tucked away and a bit remote so there was less chance someone would see us.”
There is debate in Portugal about whether authorities will go after the pair for trespassing but Thissen is adamant it was worth the risk.
“Maybe I will hear from the authorities soon but it’s the risk you take when you explore these places.
“I think I will [come unstuck one day] but as long as I’m able, I’m going to continue my adventures. I’ve done trickier locations in the past, I’ve infiltrated an active Russian military base and that one was the most frightening.
“I can’t live without it, I live for adrenalin and adventures and I think it’s good that these things get some publicity.
“It’s also part of history and they are forbidden to the public. But why? I think it’s important to show them to people.”
Thissen publishes video from his adventures on his YouTube channel, Exploring The Unbeaten Path, which has more than 92,000 subscribers.
His other discoveries in Europe have included a cold war bunker in Hungary, a deserted airport in Cyprus and an abandoned power station in Albania.