An apparatus for inserting a ground-loop for a geothermal heat-pump system comprises: a percussion mole (4); a first fluid pipe (5), connected to the mole (4), for conveying a heat-exchange fluid from the heat-pump system into the ground; and a second fluid pipe (6), connected to the mole (4), for returning the heat-exchange fluid to the heat-pump system. One of the first and second fluid pipes (5, 6) comprises an intake pipe for delivering a driving fluid to the mole (4) during insertion of the ground-loop and the other fluid pipe comprises an exhaust pipe for the driving fluid.