The distribution of the radiogenic heat sources strongly influences the geodynamics and thermal behaviour of the Earth. About 11 TW is produced by the radioactive decay of uranium (25% of the total heat flux at Earth surface), and 55% of this energy comes from the lower mantle. Here we report the first experimental evidence that aluminous CaSiO3 perovskite is the major, or even the only, host of uranium in the Earth lower mantle, since such a phase is able to incorporate up to 35 wt% UO2 (or 4 at% of U). The aluminous Ca-perovskite could be the main U-bearing constituent of a dense and radiogenic reservoir proposed in a recent model and located in the bottom half of the lower mantle.