A diamond bit rotates at the end of the drill string under gentle pressure and continuous water flush. The opening at the bit face lets a solid column of rock pass up into the barrel, where it is retained and brought to surface as core.
Standard core sizes run EX, AX, BX, NX/NQ and HX/HQ. Rods are added in three metre lengths as the hole advances, and depth is tracked by rod count. Wireline drilling recovers core through the rod string without tripping out, which is what makes deep holes economic; conventional barrels are retained for poor ground where wireline recovery suffers.