The invention concerns a device (1), which is self-powered or can be connected-up to a vehicle, for loosening the sub-soil, the device having an array of piercing tools (13) mounted substantially perpendicular to the direction of travel of the device. Before driving into the soil, the tools are oriented by means of a jointed-rod parallelogram, independently of the penetration depth selected (the penetration depth is adjustable). The position of the link rod (21) running approximately parallel to the chassis-mounted linkage between the two chassis-mounted bearings (7,8) can be altered by pivoting the piercing tool (13) or its mounting (14), against the bias of a spring (22), in a direction away from a stop (23) while the tool is executing the loosening movement in the soil.