A really deep hole. Specifically, a hole that reaches down to the Mohorovicic discontinuity, the boundary between the lithosphere and the asthenosphere. The word “mohole” is a portmanteau of the words “moho” (a short name for the Mohorovicic discontinuity) and “hole”.

In 1957 Project Mohole was proposed and in 1958 was recieved funding to dig a mohole. The aim of the Project was to return geological samples and to investigate the then-unproven theories of continental drift and plate tectonics . The Project was split into three stages:

  1. "an experimental drilling program"
  2. "an intermediate vessel program"
  3. "the final drilling to the Mohorovicic Discontinuity"

Phase 1 was completed in 1961 when five borings were drilled in the ocean floor off Guadeloupe. In 1966, with costs rising and relations between the two groups handling the Project (AMSOC and the NSF)disintegrating, the Project was terminated. Phases 2 and 3 were never completed. To this day, no moholes have ever been dug.

Though Project Mohole failed to achieve it’s main aim, the data and practical knowledge collected in Phase 1 was used in the more successful Deep-Sea Drilling Project. The DSDP was an international scientific project to drill for drilling cores of ocean sediment.

The use of moholes as a terraforming technique has been proposed. In Kim Stanley Robinson's Mars Trilogy moholes were dug in the crust of Mars to release great amounts of geothermal energy and warm the atmosphere/surface of the planet. In these books, human habitations were built into the sides some moholes so that they were warmed by geothermal energy.

Year 2000 Grolier World Encyclopaedia.
Kim Stanley Robinson's Mars Trilogy