The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in the centre of San Francisco is sinking by a few centimetres a year. Studying the city is helping scientists to improve the monitoring of urban ground movements, particularly for subsidence hotspots in Europe.

Completed in 2009, the 58-storey Millennium Tower has recently been showing signs of sinking and tilting. Although the cause has not been pinpointed, it is believed that the movements are connected to the supporting piles not firmly resting on bedrock.

To probe these subtle shifts, scientists combined multiple radar scans from the Copernicus Sentinel-1 twin satellites of the same area to detect subtle surface changes - down to millimetres. The technique works well with buildings because they better reflect the radar beam.

It is also useful for pinpointing displacement hotspots over large areas, thanks to Sentinel-1's broad coverage and frequent visits.