Smithsonian says "Not surprisingly—because the crust is thinner—some deep holes have been bored through the ocean floor. The specialized Japanese drillship Chikyu claims the record for the deepest offshore hole drilled for scientific purposes—about 10,000 feet (almost 2 miles) below the sea floor, according to James F. Allan, program director for the Ocean Drilling Program at the National Science Foundation.
The oil and gas industry also claims some deep holes, on land and offshore. BP’s Deepwater Horizon holds the offshore record. The drilling rig—lost in an explosion in 2010— managed to get some 30,000 feet below the sea, or about 5 miles."
https://www.smithsonianmag.com/smithsonian-institution/ask-s...