From the November 2006 issue of Entrepreneur

If you ever miss childhood--those carefree days of climbing trees and building caves out of sofa cushions--you'll be glad to know some entrepreneurs keep that childlike spirit alive.

Digging In
Everything old becomes new again. In the 1970s, we were reliving the 1950s through movies like Grease. In the 1980s, kids were donning 1960s fashions. In the 1990s, That '70s Show debuted. But who would have thought we would get nostalgic for the Stone Age?

David Provost, 38, has ably tapped the market for those channeling their inner Fred Flintstone. His Napa, California, business, Bacchus Caves, which he bought in 1997, builds actual caverns--underground and in hills. Initially, he was constructing them for wineries that needed climate-stable places to age their stock. But in 1999, he received his first request to build a private home cave, which can range from between $150 to $375 per square foot to create. Private caves now make up approximately 65 percent of his company's market.

This isn't to suggest people are moving their entire homes into caves; Provost has only one client who has elected to do that. "They don't really live in them," explains Provost. "People use them for other things, like a place to store artwork or have friends over. One customer wants to put in a yoga studio; another wants to store his golf cart."

Provost estimates annual sales are between $5 million and $10 million. It's a niche market, but a profitable one, and why not? Who hasn't wanted their own personal den? "One client said it really brings him back to his primal roots," says Provost. "Some of my clients are very busy people, and they describe their caves as the eye of the hurricane. They go in there, and their cell phones don't work. It's a beautiful retreat and a quiet place where they can just get away."

Branching Out
Roderick Romero has made a thriving business out of our love for leaves--he makes treehouses.

It sounds like a gig for aspiring entrepreneurs in elementary school, but Romero isn't building structures out of old plywood, cardboard, rope and hope. His tree homes can cost as much as $60,000, and they attract clients such as actress Julianne Moore and fashion designer Donna Karan. He has traveled as far as Italy and Morocco to build treehouses, and his creations are listed in the Neiman Marcus catalog. Tarzan didn't have digs this good.

Romero's business grew naturally. He built his first treehouse for an art show in 1997. He built his second treehouse for his brother in North Bend, Washington. It had internet access, and that's when Romero began to recognize the potential. "I had done four or five and was really enjoying this, and people kept calling, so I kept doing it," he says. "It wasn't any sort of conscious move to focus on treehouses. It just turned out that way."

In 2001, he started Romero Studios, which has grown 20 percent every year since. In 2007, he and his wife, Anisa, are poised to start their own furniture line and open a store near their home in New York City. Meanwhile, Romero is constructing about four treehouses a year, as well as creating immense sculptures and doing major landscaping projects for residential clients.

Romero admits he's already planning a treehouse for his 16-month-old daughter, Petra. "All my clients have some child in them," says Romero. "They start off saying, 'Oh, it's really for the kids,' but it's the adults who eventually move into the treehouse. They don't just see it as a little place for the kids to play in, but something the entire family can enjoy together."