A subsea long-distance power-transmission system (10, 34), 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 (28, 56) 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 (28, 56) by the flow of working fluid; wherein the system (10, 34) 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 (28, 56) and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct (28, 56) 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 (28, 56) 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 (28, 56) by the flow of working fluid; wherein the system (10, 34) 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 (28, 56) and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct (28, 56) 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 (28, 56) and the generating station comprises a seawater outlet in fluid communication with an opposite end of the supply duct (28, 56) to expel seawater into the surrounding sea after using the seawater to power the machine.