Mole ploughs are usually trailed and pulled by a crawler tractor, but lighter models for use on the three-point linkage of powerful four-wheel drive tractors are also made. A mole plough has a strong frame that slides along the ground when the machine is at work. A heavy leg, similar to a sub-soiler leg, is attached to the frame and a circular section with a larger diameter expander on a flexible link is bolted to the leg. The bullet-shaped share forms a tunnel in the soil about 75 mm (3 in) diameter and the expander presses the soil outwards to form a long-lasting drainage channel.