In an effort to curb Tokyo's ever-warming urban sprawl, the government is considering a massive project to cool the heart of the capital using an underground network of pipes -- tantamount to the world's largest radiator.

Sources close to the plan said funds to further study the idea in fiscal 2003 will probably be sought by the Land, Infrastructure and Transport Ministry when budget requests are submitted to the Finance Ministry later this month.

A report compiled by a panel of experts for the Japan District Heating and Cooling Association at the behest of the ministry has concluded that underground plumbing circulating waste water and seawater could help cool the city center.

A subterranean network of pipes could mitigate the so-called heat-island effect by cutting the average temperature in the immediate vicinity of the Marunouchi area by 0.4 degrees and the maximum temperature by 0.7 degrees, the report says.

The panel proposes a 6-km circuit of pipes to draw water from Tokyo Bay into the city center to help cool facilities connected to large-scale district heating and cooling systems, where water from large buildings is handled.

After the heat is exchanged between the cooler seawater and the water that has absorbed the heat of the buildings, the heat would be released into Tokyo Bay. The plan would cover 123 hectares, according to the report.

The paper also addresses a similar, smaller project of 23.6 hectares to the north of Tokyo Station using water from the Zenigamecho sewage treatment plant. Water would be cycled through pipes and used to cool buildings not yet connected to large-scale district heating and cooling systems.

The system would end the need for cooling towers in the area, where heat from buildings is conventionally disposed of via the air, according to government officials.

"The amount of heat emitted by buildings needs to be cut, especially in the heart of the city where air conditioner demand is concentrated, to cut the vicious cycle of summer warming," the report says.

The report points out that while development and loss of greenery -- major causes of the urban warming known as the heat-island effect -- have fallen off in recent years, human activity, energy consumption and heat release have increased.

While air, water, noise and waste pollution have fallen under the government's purview, thermal pollution has not. But the panel points out that the astronomical cost of the proposed project means a financial push from the government will be necessary.

The project would initially carry a 40 billion yen price tag. The cost, officials say, would be recouped through the project's benefits, such as reduced energy usage, to the tune of over 1 billion yen annually.

Thus, officials estimate the project would pay for itself in a little over 30 years.

But some question whether sending excess urban heat into Tokyo Bay might damage its ecosystem -- a concern panel members say should be further studied.

Panel chairman Toshio Ojima, a professor and the dean of Waseda University's School of Science and Engineering, downplayed such concerns. "While it might warm the Sumida River by 5 degrees, this project would only account for about 5 percent of the heat released into Tokyo Bay every year, so I don't think it would have a very big effect," he said.

Nonetheless, officials say the ecological impact of the plan and more precise calculations are needed if the idea is to become a reality.