Well the term meant for a cylindrical opening made in the earth's crust, too small for man's access thereto, the diameter of the opening being many times less than its length. The beginning of the well is called mouth, collar or wellhead, the cylindrical surface is termed the wall or hole shaft and its floor is called the bottom hole. The distance from the mouth to the bottom hole along the axis of the borehole shaft is the length of the well, while the projection of its axis onto the vertical plane represents the depth. The diameter and depth of a well vary widely depending on the purpose. Wells can be of various types, viz stratigraphic, outpost, exploratory, reconnaissance, prospecting, producing, test, injection, observation, special, etc. Mechanical devices of various types, commonly known as drilling rig, are used for drilling wells.

In Bangladesh, wells are drilled for geological, hydrogeological and exploration purposes. However, maximum number of wells are drilled for installation of hand tubewells using manual drilling method, known as sludger method. Geological and hydrogeological exploration wells are shallower in depth compared to the wells drilled for oil and gas exploration. Water wells are mostly within 300m compared to oil and gas wells of more than 3,000m depth. Shallowest wells can be less than 10m whereas the deepest well drilled in Bangladesh is of 4,977m (Fenchuganj 2) in Maulvi Bazar district. Apart from drilled holes, there are hand-dug wells known as 'dug wells' used for groundwater abstraction. These wells are of larger diameter, up to a meter or more and relatively shallow in depth (up to 15m). [Kazi Matinuddin Ahmed]