The use of the drill to probe the earth's crust, driven by primarily economic incentives, has come a long way since the first oil well at Titusville, Penn., began producing torn a depth of 21 m in 1859. Wells have now been drilled to depths of over 12 km (in the Kola Peninsula of the Soviet Union), in rocks where the pressure of pore fluid equals the weight of the entire overburden, in rocks at temperatures exceeding 400°C, and even in molten basalt in Hawaiian pit craters flooded by recent lava flows. To compensate for the hostility of such environmental extremes, drilling for resources has become one of the most robust of modern technologies.
In the late 1960s, when the ocean floors were hypothesized to have originated at the midocean ridges and to be consumed at the deep trenches, drilling proved to be the ultimate test of the revolutionary theory of plate tectonics. Now, earth scientists, confronted by problems of the evolution of the continents and physicochemical processes currently active in shaping them, have begun using drilling as one of the most valuable of experimental tools in understanding the continental lithosphere.