locked in place in contact with the drilling head, To recover the core, the drive system is disconnected, opening the top of the tube. A tool called the overshot assembly is lowered at the end of a wire using the retraction winch, when it reached the sample tube, it locks onto the top, and applying tension to the wire unlocks the sample tube from the drill head and retracts it and the core sample to the open top end of the drill tube, where it can be removed before returning the sample tube down the drill tube to lock back onto to the cutting head. This is particularly useful if there is a high risk of the hole walls collapsing when the drill tube is retracted, so it is suitable for most soil types, and to depths of 1000 m. Several diameters are available. See also Core drills (annular cutters for metal drilling) Drilling fluid Drilling rig Exploration diamond drilling References Drilling technology Ancient Egyptian technology Egyptian inventions In situ geotechnical investigations