Space surveillance shows that the ground beneath our feet is not quite as solid as it seems. Earth's crust is continuously in motion, driven by gradual geological shifting that occasionally speeds up, in the potentially destructive form of earthquakes or volcanoes.

Looking down on Earth from hundreds of kilometres away in space has proven a surprisingly accurate way of detecting slight sub-centimetre-scale motion across tens of kilometres of terrain that might otherwise go unobserved.

Combining together multiple radar images of the same location through a technique called Synthetic Aperture Radar Interferometry highlights tiny land changes that have occurred between acquisitions. It's a technique that has been used to survey the seismically active belts that make up around 15% of global land area, in order to search out previously unknown fault lines.

InSAR has also been used to survey supposedly extinct volcanoes – slight signs of ‘breathing' indicate continued active status.

ESA's GOCE mission peered deeper into solid Earth until 2013 by measuring slight variations in Earth's gravity field to detect hidden features, such as subsurface water aquifers, deep ocean currents, volcanic magma plumes and polar ice sheets.