He had drilled a hole from top to bottom and inserted a metal shaft. Boreholes were drilled by branching from a central hole. Holes are drilled between the two excavations and loaded with explosives. Several holes were drilled deep, where the temperature is a constant . Some discs are both drilled and slotted. By 1965, about 10,000 holes had been drilled at 65 locations. Wells are drilled into oil reservoirs to extract the crude oil. Up to 1970, 16 wells had been drilled near mud volcanoes. One third of these boreholes have been drilled dry. Few exploratory wells were drilled in 2005. He knew Pemex had drilled exploratory wells in the region. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face. Twenty exploration wells for hydrocarbon resources have been drilled on Prince Edward Island and offshore. In the past decades, scientists have drilled ice cores up to deep. In 2009, 36,243 wells were drilled.