A slit trench-excavating plough has an elongate frame supporting components of the plough, a number of side wall-supporters movable longitudinally, about and laterally relative to the frame, a number of wedge-shaped cutters pivoted laterally relative to the supporters. The cutters can be turned by an underground driver coupled between the cutter and the supporter to control the direction of excavation and steer the apparatus. The supporters can be moved by the drivers and rams coupled between the frame and the supporters simultaneously in opposite directions to advance the apparatus continuously.