For laying synthetic underground piping (1) a ground channel (2) is first bored and the piping (1) is introduced into the ground channel (2). To facilitate laying an underground pipe length particularly effectively, the pipe length (1) comprising a hardening synthetic mass is manufactured at the same time as it is introduced into the ground channel (2) by the prepared synthetic mass being formed continuously inside the ground channel (2) as a run of piping (3) and left to harden. A reactor (4) which can be drawn through the ground channel (2) and comprising a front reactor drum (5) and a rear double-sheathed calibrating cylinder (6) is provided for this purpose, whereby reactor drum (5) and calibrating cylinder (6) are arranged coaxially to one another and relatively rotatably and lines (8) for supply of constituents terminate in the reactor chamber (51) of the reactor drum (5) and the reactor chamber (51) is connected by way of a nozzle ring (12) to the calibrating space, open from below and formed from the double mantle (10) of the calibrating cylinder (6).