The insides of lava flows can remain at temperatures of several hundred degrees for many years due to the very low thermal conductivity of rock. Following the end of the eruption, scientists began to assess the feasibility of extracting geothermal heat from the gradually cooling flows. Experimental heating systems were soon devised, and by 1974 the first house was connected. The scheme was extended to several more houses and the hospital, and in 1979 construction began of four larger plants to extract heat from the flows. Each plant extracted energy from a square 100 metres (330 ft) on each side, by percolating water down into the hot parts and collecting the resulting steam. Up to 40 megawatts (MW) of power could be generated by the plants, which also then supplied hot water to nearly every house on the island.