To add, from Wikipedia, the deepest hole ever drilled on earth is in the former Soviet Union - note the length of time that was required - drilling
began in 1970 - it took 9 years to dig 5KM
The Kola Superdeep Borehole (Russian: Кольская сверхглубокая скважина) is the result of a scientific drilling project of
the former USSR. The project attempted to drill as deep as possible into the Earth's crust. Drilling began on 24 May 1970 on the Kola Peninsula,
using the Uralmash-4E, and later the Uralmash-15000 series drilling rig. A number of boreholes were drilled by branching from a central hole. The
deepest, SG-3, reached 12,261 metres (40,230 ft) in 1989, and remains the deepest hole ever drilled.
The initial target depth was set at 15,000 m (49,000 ft). On 6 June 1979, the world depth record held by the Bertha Rogers hole in Wa#a County,
Oklahoma at 9,583 m (31,440 ft) was broken. In 1983, the drill passed 12,000 m (39,000 ft), and drilling was stopped for about a year to celebrate
the event. This idle period may have contributed to a break-down on 27 September 1984: after drilling to 12,066 m (39,590 ft), a 5,000 m (16,000
ft) section of drillpipe twisted off and was left in the hole. Drilling was later restarted from 7,000 m (23,000 ft). The hole reached 12,262 m
(40,230 ft) in 1989. In that year the hole depth was expected to reach 13,500 m (44,000 ft) by the end of 1990 and 15,000 m (49,000 ft) by 1993.
However, due to higher than expected temperatures at this depth and location, 180 °C (356 °F) instead of expected 100 °C (212 °F), drilling deeper
was deemed unfeasible and the drilling was stopped in 1992. With the expected further increase in temperature with increasing depth, drilling to
15,000 m (49,000 ft) would have meant working at a projected 300 °C (570 °F), at which the drill bit would no longer work.