She weighs 305 tons, is 27 feet long and creeps forward in four-foot intervals, her teeth gnashing through clay and rocks like corn nuts in a blender.

You can call her Cassandra, and she's been burrowing her way 150 feet beneath Queen Anne Hill all summer.

Cassandra isn't a Tolkien-esque subterranean monster. She's basically a drill — a giant one — carving a tunnel beneath Seattle that will store storm-water runoff until it can be treated and piped into Puget Sound.

As it is, Seattle's sewage and drainage system is woefully inadequate — overflowing pipes to the point where a mix of sewage and rainwater spills into surrounding lakes and the Sound as many as 50 times a year in certain places.

Many of the pipelines were built near the turn of the last century and simply aren't equipped to handle the residential waste and storm runoff of a modern big city.

King County and Seattle have been tackling the problem since 1986, and the tunnel is an essential piece of that push toward a cleaner, more efficient system.

"Most of the systems were built in the days of the horse and wagon," said project manager Judith Cochran, with the King County Wastewater Treatment Division. "Those same systems 100 years later begin to look pretty out of date."

Almost a quarter of the 6,200-foot tunnel is finished, with the line running roughly below Mercer Street. Estimated completion time is in 2004 and, at 14 feet 8 inches in diameter, the concrete-lined tunnel will hold as much as 7.2 million gallons of waste water.

Workers first dug a 66-foot-deep hole along Elliott Avenue West and lowered Cassandra by crane. A rotating disc at her nose bares rows of metal teeth to chew sideways into the earth.

Machines at Cassandra's rear line the tunnel with 9-inch-thick concrete rings that are bolted together to form the tunnel itself. There is no danger of the tunnel collapsing on workers, since Cassandra supports the exposed earth around it.

The machine moves about 4 feet every 40 minutes, spending 20 minutes clawing at the earth and another 20 assembling the tunnel lining. That's 40-plus feet per shift.

About 47 cubic yards of dense clay and grainy glacial till are removed for every 4 feet Cassandra travels. The material is run out on a conveyor belt to carts that carry it to the mouth of the tunnel.

The county is building tributary pipes to direct waste water to the larger tunnel, and a new treatment plant near the tunnel's entrance will supplement the plant in Magnolia by filtering, disinfecting and de-chlorinating the water so that when it enters the Sound it will be safer for humans and healthier for marine life.

Lisa Heyamoto can be reached at 206-464-2779 or at lheyamoto@seattletimes.com.