On my first afternoon in Copenhagen, I met with Jens Falkenberg at a restaurant on Dag Hammarskjölds Allé, in an affluent part of town. Falkenberg is a 58-year-old roof salesman. He first heard about Madsen years ago, when he saw a segment about him on television and, by coincidence, met him the next day at a diving shop. He started volunteering at his workshop and helped build the Nautilus. He told me that the police had been calling, asking about a saw that was missing from Madsen’s rocket workshop.

If something did not please Madsen, “he would behave like a child who just lost his toy.”

Falkenberg was like many of the others who volunteered with Madsen, who called himself “a maker of extreme machines.” They spent their weekdays in regular jobs but were weekend builders. They wanted the feeling of community the workshop gave them. At the center of their alternate universe where men built submarines and rockets was Madsen himself.

Some volunteers talked about Madsen as a generous spirit, the kind of guy who would invite a friend who was feeling down “to take part in his little adventures as a means of cheering him up,” as a friend named Lars put it.

Others reexamined old incidents and behaviors. Madsen could swing between rage and euphoria. One volunteer at Copenhagen Suborbitals told me that if something did not please Madsen, “he would behave like a child who just lost his toy or dropped his ice cream or something.” When his mood turned, “most people would know what was going to happen, so they would stay away from him before stuff started flying.” Volunteers said Madsen threw hammers, screwdrivers, and other tools. One volunteer, who asked to be identified by his initials, S. W., helped build the Nautilus. He recalled how Madsen would go from being supportive to “pensive, jubilant, exasperating, and sarcastic.”

“It’s hard for us to understand what drives a madman, because we are not mad,” Falkenberg told me. He then described a recurring joke: Madsen would pretend to be a violent Nazi and would mime hitting Falkenberg, saying “Should I punch you in the kidneys?” or Madsen might joke: “What if I inject battery acid into your veins?”

There was also a lot of joking around about Nazis in the workshop. Crewmembers called each other by Nazi-inspired nicknames. Madsen was called Kaleun, for Kapitänleutnant, a nod to the 1981 film Das Boot, about a fictional German U-boat unit during World War II, Falkenberg said. When they went out in the sub, the crew spoke German, reciting lines from the film.

Madsen’s fascination with space and rockets and technology could hoodwink you into thinking he was a man of the future; you could miss the fact that his obsession was rooted in nostalgia. He was enamored with the early Apollo missions in American space exploration. The reverence he held for the Third Reich was hard to detect as it was framed as irreverence, but it was there. “Some of the way the Nazi regime worked, they did horrible things and they should be executed and everything. But some of the things they did, it worked,” the former workshop volunteer told me. “They built the biggest military machine in just four years. They built it almost out of nothing.”

Building something out of nothing was central to Madsen’s philosophy, as was his belief that he should be able to play by his own rules and control his own destiny. He looked down on people for being cautious. He talked about wanting “to be free from authorities” in making his submarines. After he left Copenhagen Suborbitals, he kept a blog about the progress at Rocket Madsen Space Lab. In one entry from 2015 he described his team as people who “all know that they are taking part in a Peter Madsen project, just like they would do if it was a von Trier movie ... the unqualified belief that Madsens crasy [sic] dreams tend to become reality … makes these people invest time and money.”

Windmills on the water. Mustafah Abdulaziz

I had been in Copenhagen a week when I went looking for a woman I knew did not want to talk to me. She was a friend and recent sexual partner of Madsen’s. She lived in a converted building in Refshaleøen. One afternoon I walked through its vast hallways until I managed to find her room. I knocked on her door, and she let me in. I had twisted my ankle on the way over and was limping. She let me sit on her carpet and keep my injured foot raised while she ate toast. Her eyes seemed heavy with sleep.