A subsea long-distance power-transmission system , comprising: an electrically-driven pumping station arranged to produce a flow of pressurised working fluid ; an electricity generating station having an electrical generator coupled to a fluid-powered machine , the generating station being situated underwater; a supply duct extending across the seabed between the pumping station and the generating station , that duct being arranged to convey the flow of working fluid from the pumping station to power the machine ; and a subsea installation connected to the generating station to be powered electrically by energy conveyed along the supply duct by the flow of working fluid ; wherein the system is arranged as: a closed loop, and further comprises a return duct extending across the seabed between the generating station and the pumping station , that duct being arranged to convey the flow of working fluid from the generating station to the pumping station for re-pressurisation; or an open loop, wherein the working fluid is seawater, the pumping station comprises a seawater inlet in fluid communication with one end of the supply duct and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct to expel seawater into the surrounding sea after using the seawater to power the machine . an electrically-driven pumping station arranged to produce a flow of pressurised working fluid ; an electricity generating station having an electrical generator coupled to a fluid-powered machine , the generating station being situated underwater; a supply duct extending across the seabed between the pumping station and the generating station , that duct being arranged to convey the flow of working fluid from the pumping station to power the machine ; and a subsea installation connected to the generating station to be powered electrically by energy conveyed along the supply duct by the flow of working fluid ; wherein the system is arranged as: a closed loop, and further comprises a return duct extending across the seabed between the generating station and the pumping station , that duct being arranged to convey the flow of working fluid from the generating station to the pumping station for re-pressurisation; or an open loop, wherein the working fluid is seawater, the pumping station comprises a seawater inlet in fluid communication with one end of the supply duct and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct to expel seawater into the surrounding sea after using the seawater to power the machine . a closed loop, and further comprises a return duct extending across the seabed between the generating station and the pumping station , that duct being arranged to convey the flow of working fluid from the generating station to the pumping station for re-pressurisation; or an open loop, wherein the working fluid is seawater, the pumping station comprises a seawater inlet in fluid communication with one end of the supply duct and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct to expel seawater into the surrounding sea after using the seawater to power the machine . <EOS>