Tokyo is an urban planner's nightmare. Just about every nook and cranny is filled with narrow, twisting roads, crowded neighborhoods and people, people, people.

So officials are beginning to exploit a new frontier - deep underground.Bolstered by an arsenal of technological advances that allow construction at far greater depths than in the past, government planners envision an array of projects from better water and gas mains to tunnels for highways and high-speed trains.

"The time for this sort of construction has come," said Yoshitaka Kuwabara, an official at the National Land Agency.

Kuwabara said new technologies have opened the way for construction as far as 330 feet beneath the earth. Conventional tunnels for utilities or subways usually run at depths of no more than 35 feet.

Tokyo's dive down has already begun.

The first of the capital's deep subterranean projects - a highway tunnel running 66 feet below the floor of Tokyo Bay - was completed in April 1997.

One of the biggest obstacles to deep tunneling was the increased danger of cave-ins.

Engineers have overcome that with cylinder-shaped "shield machine" robots the size of a house that use a combination of rotating teeth and high-pressurized water to slowly bore through the ground.

Workers follow behind, stabilizing the tunnel with fast-drying concrete and loading the loosened dirt and rocks into dump trucks to take to the surface.

The world's biggest shield digger, almost 50 feet in diameter, is now cutting a tunnel beneath Tokyo that will capture overflow from the city's rivers and prevent flooding.

Going deep is allowing planners to get around another persistent headache in Japan - obtaining permission from landowners to dig under their property.

Kansai Electric Power in Osaka, Japan's second-largest city, found private landowners so demanding that it decided to avoid them altogether.

It is building a tunnel for high-voltage cables in the city's crowded downtown by threading it far below the maze-like streets. The company uses advanced ultrasound technology to keep track of the exact location of the construction work 220 feet below ground.

To make such projects easier in the future, Japanese officials say they will propose a law that would end land ownership at a point 132 feet beneath the surface. Anything below that would be in the public domain, opening the way for companies to begin construction with only a public permit.

"We can avoid all the hassles of surface or shallow subterranean building," said Masahiro Iwatsuki, one of the officials drafting the bill.

Technology is being used to address other problems with underground structures.

Engineers who built the four-lane Aqualine freeway beneath Tokyo Bay worried the 6-mile-long tunnels would become a deathtrap if a fire broke out. Their answer was an innovative escape system that allows motorists to slide down emergency chutes to a separate walkway beneath the road with its own air supply.

Surprisingly for many people, a benefit of deep subterranean construction is that it's safer during a strong earthquake than tunnels at shallower depths.