The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5-5 km, and at temperatures of 450-600 degrees C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550-650 degrees C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000-5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the "pipe". Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 degrees C. A deep well producing 0.67 m(3)/S steam (similar to 2400 m(3)/h) from a reservoir with a temperature significantly above 450 degrees C could, under favourable conditions, yield enough high-enthalpy steam to generate 40-50MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide. (c) 2004 CNR. Published by Elsevier Ltd. All rights reserved.