In principle, the expansion of the universe can be harnessed to provide energy. In a gedankenexperiment, energy is gained by connecting together widely separated bodies with strings. The tension and the energy generated are calculated for single strings. Mining energy in an expanding universe in this way raises unresolved issues concerning the conservation of energy. Apparently, the tethered-body experiment delivers "nascent" energy that previously did not exist in any identifiable and quantifiable form. It is argued that energy in a homogeneous and unbounded universe, in general, is not conserved on the cosmic scale.