Editors' Summary: In recent years, environmental law has gone underground. The realization that groundwater has been widely contaminated by toxic substances from chemical dumps, pesticide application, and other sources has focused attention on subsurface pollution. Recently, a new culprit has been identified—leaking underground tanks in which oil, gas, chemicals, and wastes are stored. Congress perceived the problem as serious enough to warrant addition of a new program to the Resource Conservation and Recovery Act (RCRA) at the end of last year. The authors examine the problem of leaking underground tanks, Congress' far-reaching response in the new Subchapter IX of RCRA, and the Environmental Protection Agency's (EPA's) initial efforts to implement the complex requirements. They highlight the special challenges posed by the fact that EPA must regulate a vast community of tank owners, many of whom have been untouched by earlier federal pollution control laws, and suggest a number of ways in which EPA and the regulated community can improve the chances of success.