The first of two patented, government-owned subterranian drilling machines

September 26 1972

They melt rock via various means, and use it to form smooth tunnel walls. First tested in New Mexico.

See full information of this patent: U.S. Patent No. 3,693,731 dated Sept. 26, 1972
 

Other Subterrene patent

27 May 1975

See full information of this patent: U.S. Patent No. 3,885,832 dated May 27, 1975