"In the mid-80s I heard of a "Project Thunderwell" from someone who had
been employed at Livermore. In about 1991, I asked Dr. Lowell Wood
about it, who was (and presumably still is) a prominent weapon
physicist at Livermore. He told me that it was a project, never
actually constructed, to launch a spacecraft on a column of
nuclear-heated steam. The idea was that a deep shaft would be dug in
the earth and filled with water. A spacecraft would be placed atop this
shaft, and a nuclear explosive would be detonated at the bottom." -
Carl Feynman

http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html