Abstract: A system for drilling a subterranean bore at a subsea location is provided, the system comprising a drilling unit for location on the seabed and remotely operable from the water surface to drill a bore using a drill string; a transport assembly moveable between the water surface and the drilling unit when at the location on the seabed, wherein the transport assembly comprises a receptacle for receiving a downhole tool; first transfer means for transferring a first downhole tool from the drill string to the transport assembly; and second transfer means for transferring a second downhole from the transport assembly to the drill string; whereby the first and second transfer means are operable to recover from the drill string a first downhole tool and deliver to the drill string a second downhole tool in a single deployment of the transport assembly from the water surface to the drilling unit.