German ESA astronaut Matthias Maurer practices Space Station repairs in the Neutral Buoyancy Lab at NASA’s Johnson Space Center in Houston, USA. The giant pool is over 12 metres deep and features a full-scale mock-up of the International Space Station. It is used to train astronauts in the very precise and difficult task of making repairs or installing new equipment in the vacuum of space. To simulate microgravity underwater, astronauts are weighted down to minimise the buoyant force that would otherwise have them bopping back up to the surface. They must also work very slowly to compensate for the drag caused by water.