BY San Jose Mercury News

To celebrate his wife's 38th birthday this month, Todd Darling secretly orchestrated what he called her "day of adrenaline." First, he and Kathleen raced go-carts in Burlingame. Then the San Francisco couple headed across the bay to sky dive.

Indoors.

Without a parachute.

Welcome to the latest in extreme-sport thrills in the Bay Area: a $6.5 million vertical wind tunnel at iFly SF Bay in Union City. Though intended to simulate a sky diver's free-fall, massive fans designed to ventilate skyscrapers actually send your body aloft on a cushion of 120-mph-plus air.

When Kathleen Darling emerged after hovering precariously inside the 12-foot-wide hexagonal glass tunnel, she carried away a memory that ranks with a bungee-jump in college - but without the panic attack.

"I thought I would feel like I was falling, and I didn't feel like I was falling at all," she said, oblivious to the raccoon marks her goggles had pressed into her face. "I felt like I was flying. You know, floating. It was amazing."

"If we had time," she added, "I'd go a couple more times, for sure. I want to learn how to do a flip."

It takes only a few moments aloft to understand Darling's reaction. Unlike sky diving - or even the "Drop Zone" free-fall ride at Great America in Santa Clara - there's no sense that you're plummeting earthward, no stomach thrusting up your throat. Neither is there reason to worry that your parachute will fail to open

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