In Lleida, a two-hour drive west of Barcelona, up a sandy hill on the southern edge of town, past a 12th-century castle, there's an army barracks. The three H-shaped buildings were built when Spain was fascist, ruled by Francisco Franco, and they look it: squat, beige, institutional. But inside, things are changing. For the past two years, Barcelona architect Felipe Pich-Aguilera has been converting the buildings into an agricultural research center for an organization called PCiTAL. The H's are lined up with their slots facing each other and he connected them with a set of greenhouses. Inside, it's light and cool and plant-smelling—an oasis in a shell of autocracy.

Pich-Aguilera is building on haunted ground. Before any Spanish architect talks about his own work, dead spirits have to be invoked, like a séance. "I was born in Figueras," begins Enric Ruiz-Geli, another Barcelonan. Figueras is northwest of Lleida, near the coast. "That's where Dalí was from. Duchamp, Picasso, Miró all worked there. They say it's a very creative place." Why? "It's very windy," he says.

In a Luis Buñuel story (he spent summers there), the wind is a magician who turns priests into umbrellas and houses into mountains. Dalí was inspired by the weird clouds it whipped up over the valley. This is where surrealism was born, as artists got tipsy on the heady cocktail outside: mountains, deserts, sun, waves, wind. Today, Spanish architects are joining in, turning those forces into some of the most radical structures around, from Pich-Aguilera's building that breathes to offices that float by Juan Herreros and a house that squints in the sun by Ruiz-Geli. In Spain, they don't seem that farfetched.

"There's an ideological tendency towards the organic in Spain," Terence Riley, director of the Miami Art Museum and curator of a seminal show of Spanish architecture at MoMA in 2006, explains. "It implies a greater understanding between environmental and constructive processes." In other words, between how nature works and how buildings work, and how they work together.

Pich-Aguilera calls this "bioclimatic architecture." On the phone while walking through Barcelona, he breathlessly explains, peppering his monologue with Corbusier quotes in perfect French. "Bioclimatic means the architecture is a machine to produce climate," he says. "La maison est une machine, but a machine to produce climate without fuel or natural resources." In Lleida, it works like this: that famous Spanish wind blows over adjustable vents in the greenhouse roof, cooling the air inside and pushing it through the rest of the building. "We asked, Can we re-use the components in the future, consume less energy, and put out less CO2?" he says. "They're quite specific parameters."

Juan Herreros, from Madrid, would call Pich-Aguilera a fanatic, and he'd be right. In 2004, fed up with having to import energy-efficient technology like insulating curtain walls from cooler, cloudier Northern Europe, Pich-Aguilera designed his own kind of wall for Spain's sunnier climate out of colored polycarbonate sheets and air-conditioning insulation, and installed it on an office building in Barcelona. Pich-Aguilera is proud to be called a green architect, but not Herreros.

"I'm not a sustainable architect," he insists. "I'm an architect." Out of Madrid's rigorous polytechnic university, he cuts his interest in organic processes with a strong dose of nuts-and-bolts pragmatism. It's a Madrid thing. "Here, the school is very structured," he says, "but in Barcelona, architects had to create their own structures. It's a collection of outsiders, but we're like a well-organized army." For all his humble, common-man posturing though—"normal isn't so bad," he says—Herreros is and always was a bit of a radical.

He started practicing in 1984 with Iñaki Ábalos, in glimmering dawn of Spain's architectural golden age. Franco died in 1975, but a recession in the '80s kept things cool until the Barcelona Olympics in 1992 brought an overdue building boom. New architecture opened up its parks and plazas, pulling the city out of fascism. Richard Meier's glassy contemporary art museum brightened up the Raval neighborhood while SOM and Frank Gehry worked on the waterfront. In 2000, Ábalos and Herreros joined in with the Barcelona Ecopark, finished in 2004: a sliver of green next to an industrial plant on the coast that reanimated a dead part of the city by connecting it with the ocean. "Sustainability reminds architects of context," Herreros says. "The site, the atmosphere, the world."

The two split in 2008. Their work was always sustainable—built from recyclable materials, energy efficient, the usual—but Herreros's solo projects are more explicitly naturalistic. Take a competition entry he designed in '07, just before the breakup. A research center for the fusion power R&D organization ITER in a forest outside of Marseilles, it fingers out through the trees, avoiding the big ones, its floor hovering just above the ground and its roof just below the treetops, suspended in midair.

This is low-impact architecture, designed to let the forest grow around it. It's poetic, but Herreros is an engineer at heart. "Sustainability is a technology, like bricks or steel," he says, and in the ITER project, sustainability concerns were technical concerns, and vice versa. "In our first meeting," he explained, "Every time one person had an idea, someone else would shout out the numbers. ‘We can put a solar panel in the parking lot,' I'd say, and then someone would go, ‘You need this many square meters.' Or, ‘let's raise the building up on legs to let the forest grow underneath.' ‘That'll save you this many millions in foundation cost.'"

Yes, Herreros is hung up on construction, but he can't help it. "Spain is a nation of builders," says Riley. "There's a rigor, even in the most organic architecture. These aren't dreamers who can't realize their work. There are outrageous ideas, but they're matched with outrageous technique." It's been like this at least since Antoni Gaudí, whose wild, wavy facades snuck up on turn-of-the-century Barcelona like fever dreams. His heir, bringing the same surreal extravagance and technological chops to today's scene is Enric Ruiz-Geli.

"When things look impossible, it's good to walk by Gaudí's buildings and see what he struggled with," Ruiz-Geli says. "It's like a vitamin." Ruiz-Geli takes his office on field trips to Gaudí's 1910 opus, Casa Milà, just a few blocks away. (That's the one with the rippling stone front and weird mask-like sculptures on the roof.) Casa Milà dances with the city and that dialog—architecture as performance—is Ruiz-Geli's biggest inspiration. His studio's first project, in 1997, was a fireworks show in Figueras, his hometown. "There were 30,000 people watching us drawing in the sky," he said. "We made glowing architecture in the middle of the night."

On the phone in his car, Ruiz-Geli talked about Villa Nurbs, a house for a family friend in Empuriabrava, a ritzy seaside suburb an hour and a half north of Barcelona, and just east from Figueras. It'll be done this winter. Its cartoony name comes from a way of modeling complicated curves on a computer—like the ones on a Gaudí building, or this house's bubbling roof. The roof is made of oblong plastic balloons, transparent but printed on each side with a pattern of dots. When the balloons fill with air from a computer-controlled pump, the dots come apart and sunlight floods the house. Let out the air and the dots line up: instant sunblock.

"The neighbors said, ‘I didn't know this technology was here already,'" Ruiz-Geli says, sounding exasperated at the traffic, the neighbors' ignorance, or both. "And I said, ‘come on! You have a computer in your car, but you go in your house and there's no computer, nothing.' My architecture is the future."

His latest project is the Hotel Forest in southwest Barcelona, opening next year. It's draped in a web of solar-powered LEDs that suck up sunlight during the day and shoot it back out as colored light at night. Ruiz-Geli wanted an element of chance, so the LEDs emit a spectrum, from white to dark red, based on how sunny the day happened to be. The building is performing, and the weather writes the script.

In Spain, the wind can make giants out of windmills or mountains out of houses, and the sun can change the color of a building. The best architects know that, and by submitting to that unpredictable force—by letting wind be the air-conditioning in Lleida, or the forest be the foundation in Marseilles—they make architecture both Earth-bound and otherworldly. It's about combining high tech methods and natural science with faith in the unknown. "Phenomena—climate, heat, humidity, wind—and fiction," Ruiz-Geli says. "And fiction is freedom."