Inside a huge magnet sits a hollow cylinder, like a bell. If an axion, a dark matter particle, flies nearby, the bell should ring softly. But ordinary copper, from which the bell is made, conducts high-frequency oscillations poorly: current flows only on the surface, and much energy is lost. A superconductor would ring more cleanly, but in a strong magnetic field, vortices pierce it, and it stops working.

The moat works like this: the gaps between parts of the bell become narrow channels where the wave rapidly attenuates and cannot escape. Thanks to this, the bell became a hundred times better in quality and could search for dark matter eight times faster. Now physicists can listen for the faint ring of axions in an 8 tesla field, and the moat around the bell reliably keeps the signal inside.