When salty ocean water flows through Earth’s magnetic field, an electric current is generated, and this in turn induces a magnetic signal. However, the field generated by tides is tiny – and extremely difficult to measure, but ESA’s Swarm mission has done just this in remarkable detail. Swarm has been used to measure the magnetic signals of tides from the ocean surface to the seabed, which offers a global picture of how the ocean flows at all depths. The magnetic tidal signal measured by Swarm is important for ocean and climate modelling, and is used to determine the electrical properties of the Earth’s lithosphere and upper mantle.