While American and Soviet astronauts battled over Earth's orbit, the countries' engineers fought a lesser-known battle deep underground. The goal was to drill to the Mohorovicic Discontinuity, the theorized boundary between the Earth's crust and its magma-filled mantle. The United States began its effort in 1961, drilling into the shallow crust under the Pacific Ocean off the Mexican coast. Known as Project Mohole, the endeavor failed to achieve its objective before shutting down in 1966 due to a lack of funding. Seeing an opportunity, the Soviet Union began its own drilling project in May 1970, on the Kola Peninsula in far northwest Russia. When drilling ceased 22 years later, the Soviets' Kola Superdeep Borehole was the deepest hole on Earth.
As one might expect, drilling to these depths was no simple task. It took nine years just to surpass the depth of the world's previous record holder, the 31,440-foot (9,583-meter) deep Bertha Rogers well in Oklahoma. When the drill reached 39,000 feet (11,887 meters) in 1983, the Soviets decided to take a year off to celebrate their achievement. Soon after the project resumed, however, the drill broke about 16,000 feet (4,877 meters) above the bottom of the hole. This forced the team to restart just above this broken section, and they continued to bore until extreme temperatures -- as high as 570 degrees Fahrenheit (299 degrees Celsius) -- made drilling impossible [source: Santoski]. Officials called off the project in 1992, after reaching a final depth of 40,226 feet (12,261 meters) [source: Madrigal].