Sometimes an idea is so elegant that it really deserves to work. One such is a proposal put forward by the boss of Deep Fission, an aspiring nuclear-power firm in Berkeley, California. Elizabeth Muller’s brainwave is to build a reactor at the bottom of a mile-deep shaft drilled into Earth’s crust, and then fill the shaft with water. This would, in one fell swoop, minimise the risk of radioactive leaks, dispose of the “hot” waste reactors generate and eliminate much of the paraphernalia that make them expensive to build and run.