On a recent evening I joined a group of strangers on a twilight tour of San Francisco’s Yerba Buena Island. This wasn’t your typical urban outing. First, consider our guide, an artist named Eugene Ashton-Gonzalez. He’s wearing a tattered nautical jacket and a two-day beard. He’s confiscated our cellphones and placed them in a heavy briefcase.

Then there’s our stealthy, guerrilla-style approach. Ashton-Gonzalez leads us across a small beach, around a clump of trees and up a steep trail. As we pass rows of empty military homes, he orders us to take cover while a private security guard passes nearby. We’ve entered a restricted area that is scheduled for demolition.

“Step lightly,” he whispers.

At the top of the island we dart across a small parking lot to reach our destination -- a lonely wooden tower. Ashton-Gonzalez delivers us through a secret passage and up a darkened staircase to the top floor.

That’s when our collective jaws drop. It’s as if we’ve entered the bridge of a giant ship anchored grandly in the middle of the bay. Jutting beneath us are the white towers of the Bay Bridge. And shimmering all around are the lights of San Francisco, Oakland, Berkeley and beyond.

Welcome to the Signal Room. It’s a rogue museum and a modern-day speakeasy, except that the illicit part of the experience is not what you drink, but how you get in.

Ashton-Gonzalez created the Signal Room last year after returning from a few years in New York City. He was moonlighting as an actor and storyteller, and conspiring with a group of urban explorers and trespassers. Their rogue pop-up projects included running a bar in an abandoned water tower in Manhattan.

Ashton-Gonzalez was looking to replicate his New York experiences back in his hometown. That’s when he discovered the abandoned radio tower on Yerba Buena Island and began researching its history. It was built by the military in 1917 to direct ships through San Francisco Bay. Later, it guided aircraft during World War II and served as an officers club. After the Loma Prieta earthquake struck in 1989, the building was sealed shut. Until Ashton-Gonzalez found a way to sneak inside.

Working with a small group of artists and urban explorers, he repaired and resupplied the tower. He installed tiny exhibits detailing the island’s history -- as an Indian burial site, as a smugglers’ haven and as a listening post for the military. He’s also staged dozens of secret events -- from small concerts and poetry readings to panel discussions and plotting sessions -- making this location the most exclusive (or certainly the most restricted) nighttime venue in the Bay Area.

Events here are free (unless food is being served) and typically involve 15 or 20 people who are invited by previous guests. Eugene says that protects against what he calls "cool hunters." On my visit I nibbled on plates of smoked lamb, while a group of storytellers recounted old family yarns and Oakland-based musician Alycia Lang sang her tender songs.

Ashton-Gonzalez says the tower’s wooden construction, combined with traffic noise from the Bay Bridge, provides an acoustic camouflage that protects his events from the scrutiny of police and private security (so far not a single happening has been raided).

All this cultural commotion caught the eye of San Francisco writer Gary Kamiya, who visited the tower earlier this year.

“When you see a site like that, you cannot be intellectually prepared for it,” he says. “It’s so stunning.”

Kamiya says the work of Ashton-Gonzalez and others is proof that an underground countercultural scene is still vibrant in San Francisco in spite of a tech boom that has driven many artists from the city. He says it’s a tradition that began decades ago with groups like the Suicide Club and Cacophony Society. They used tunnels, bridges, sewers and civic buildings as a kind of illicit stage set.

“That’s why people like Eugene describe themselves as 'experience designers,' ” Kamiya says. “It's not that you're going to design a play or a piece of music. You're going to design an experience. And ideally it will be an intense experience. So when you go up into an abandoned old room with one of the greatest views in the world with a group of strangers, you've succeeded in doing that.”

Another fan of the Signal Room is John Law. He joined the Cacophony Society in the 1980s and was a co-founder of Burning Man (he later broke with the festival).

“To me the whole thing about urban exploration -- and the best thing about it -- is that it's a connection with the past and it's really ephemeral,” Law says. “And Eugene, finding this space and understanding what it was and giving it a last breath of life, that's brilliant.”

At this point you’re probably wondering something. If all of this is such a big secret, why is Ashton-Gonzalez talking about it to a reporter?

“I want to save the building,” he says.

Preservation isn’t what motivated him at the beginning. He admits that when he first discovered the tower all he wanted to do was have a huge party. But he’s come to see the space as something more than a speakeasy on life support. It’s a San Francisco landmark, albeit a hidden one.

“This place seems so uniquely special,” he says. “Could you put a little museum up here? Could you put a little exhibition space for public art, street art, sculpture?"

Recently, Ashton-Gonzalez assembled a team of architects and began a quiet lobbying effort to save the building.

“It’s sublime in the truest sense,” says Bruce Tomb, a San Francisco-based architect who helped transform a former Army outpost in Marin County into the Headlands Center for the Arts.

“The view is priceless. And to have an artifact in that location is an opportunity to talk about our heritage.”

Ashton-Gonzalez says he brought city officials to the tower but he declined to identify them.

“They were pretty much to a man and woman quite impressed,” he says, but “a little curious about the bureaucratic jumps and hoops that would be required” to save the tower.

Officials from the Treasure Island Development Authority confirmed that the tower is slated for demolition in May as part of a plan to bring hotels and luxury condos to the twin islands. With that in mind, Ashton-Gonzalez is stepping into the spotlight, knowing that more publicity could bring an end to his secret gatherings.

But he doesn’t care. He says it’s time for all of San Francisco to share the story of the Signal Room.

Eugene Ashton-Gonzalez is a transmedia storytelling artist and director of Eat My Heart Out [eatmyheartout.org] , a private storytelling dinner party for KWMR community radio in West Marin.