Sparks from a welder's torch touched off a billowing fire behind the state Capitol Monday, destroying an underground air conditioning unit and shed.

Damage may be as high as $100,000, officials said.Flames burst 30 feet into the air during the blaze that erupted about 3:40 p.m. when a redwood shed housing the unit caught fire.

A heavy plume of black smoke attracted hundreds of onlookers and stopped pedestrians throughout downtown.

Concrete walls contained the fire to the air-conditioning vault, and metal doors in an access tunnel prevented smoke from traveling into the Capitol, Battalion Chief Michael Andrews said.

The shed and unit sat below ground-level against the parking terrace between the Capitol and state office building. Firefighters sprayed water down through a metal grating on top of the shed to control the blaze.

"The only way to access that area is through a utility tunnel, and we would have had to drag hoses 500 feet to get there," Chief Dennis Sargent said.

Neither the state office building nor the Capitol was evacuated.

Intense heat from the fire melted the inch-wide grating covering the air-conditioning vault, but no injuries were reported.

More than 30 firefighters and a half-dozen engines responded to the blaze at 3:50 p.m.

The air conditioning vault was one of two main units that cooled the state office building and Capitol. It was being modified when the fire started.