WASHINGTON – Fifty feet below ground and steps from the U.S. Capitol, workers haul bricks and cement, lay marble and tiles, and put other finishing touches on a subterranean project almost as large as the building itself.

The three-level catacomb will be a place to welcome visitors and make lawmakers safer in doing their business.

To critics, it stands as a half-billion-dollar monument to Washington excess.

Slated to open in September 2006, the Capitol Visitor Center will be the ninth addition to the building in its history – nearly doubling its size.

To make room above ground for the center’s entryway and an outdoor plaza, some 300 parking spaces were relocated and 68 trees chopped down, including six memorial ones, the oldest a pin oak planted in 1917 to honor Kansas Rep. Joseph Taggart. Eight memorial trees were moved elsewhere, and 85 new trees planted.

Inside the center, an exhibition gallery twice as big as the Capitol Rotunda, the room beneath the dome, will aim to demystify Congress for thousands of people who tour the building daily.

The idea of a visitor center had been around since the 1970s, but it wasn’t until the fatal shootings of two Capitol police officers in 1998 that the project took on momentum.

Congressional auditors have put the center’s cost at $515 million.