It's hard to miss the 10-story-high cube that appears to be balanced precariously above the bustling freeway in Santa Ana.
For more than a decade, the metallic cube has loomed outside Orange County's Discovery Science Center, a strange roadside attraction that prompts people to pull off the Santa Ana Freeway to get a closer look.
And for more than a decade, visitors have asked the same thing: what's inside?
"It's one of the biggest questions I always get," said Joe Adams, president of the science center and museum. "My answer to date has been ‘nothing.' "
The cube has long been empty, an attention-getting facade for a museum whose interactive exhibits are housed in the more conventional concrete building to which it's attached.
This week, the science center is installing its first exhibit inside the upended cube: a 12,000-pound rocket engine.
For the first time, visitors will be able to walk into the cube and look at the underside of a liquid hydrogen- and oxygen-powered engine used in Delta IV rocket test launches.
The exhibit is the latest in a long run of sometimes strange, occasionally controversial exhibits in the museum. There's a fossil dig area, an exhibit devoted to the science of hockey, an earthquake zone where visitors can set off a quake, observe liquefaction and experience the shaking inside a wooden house.
Four years ago, the center opened an outdoor exhibit where visitors would wander inside the guts of a dinosaur, empty the beast's bladder with a pull of a rope or set its heart pounding by twirling a crank. There was no way to be sure the dinosaur's innards were anatomically correct, but visitors seemed awed nonetheless.
Less successful was the 2008 plan to have an artist encircle an Asian elephant in an enormous soap bubble. It was meant to be an attention-getting stunt in advance of the museum's annual Bubblefest.
The event was canceled when animal rights activists protested, calling it a cruel and demeaning stunt.
Filling up the museum's cube has proved to be far less controversial, but perhaps more challenging.
A wide-load truck transported the massive RS-68 engine, donated by Pratt & Whitney Rocketdyne, from Canoga Park to the science center last week. Cranes suspended it inside the cube.
The 25-foot-tall piece of rocket equipment was a tight fit. Crews had to maneuver it into the structure with only 4 inches to spare.
When the Boeing Rocket Lab exhibit officially opens July 2, visitors will be able to stand under the nozzle of the rocket engine and press a giant red "launch" button, starting a simulated blastoff sequence with smoke, heat lamps and footage of rocket launches.
"Of course if you were really there, you'd be incinerated," Adams noted. "But if you're here, you'll be able to see what it's like to be in the firing line of a rocket."
The display will be surrounded by hands-on exhibits about rocketry, including stations where people can launch air- and water-powered model rockets into the upper reaches of the hollow cube.
Adams said the science center has long been waiting for the perfect item to display in the cube. When the prospect arose of housing an engine used to launch communications and weather satellites into orbit, Adams said, something clicked.
"We started thinking about it and thought, wow, what a really cool piece that is not only of part of the past but also the workhorse engine of the future," he said.
To obtain the object, the science center had to get the delivery route approved by Caltrans and get waivers from the U.S. government pledging that it wouldn't need the engine returned for defense purposes.
Museum administrators promise that it will be kept in the peace and safety of Santa Ana for many years to come.