Triton workers excavated a 235 feet ( 72 m ) shaft , known as Borehole 10 - X .
Triton workers supported by a steel caisson to bedrock , in 1971 .
Triton workers excavated a 235 feet ( 72 m ) shaft , known as Borehole 10 - X and supported by a steel caisson to bedrock , in 1971 .