Subsidence

ISPRA, Italy's environmental protection institute, defines it as any vertical lowering of the ground surface, whatever the cause. Natural causes include movements of the Earth's crust and sediments compacting under their own weight. Human causes include excessive pumping of groundwater, the extraction of gas and oil, and land drainage schemes. In Italy it mainly affects the Po and Venetian plain and many coastal plains; Venice and Ravenna are the best-known cases.

For a building, how evenly the ground moves matters a great deal. Sinking of a few millimetres a year is common on plains and often causes no problems. If the ground sinks at different rates over a short distance, though, the structure is put under more strain and cracks can appear.

Subsidence can also be measured from space. The report uses the European Ground Motion Service (EGMS) of the Copernicus programme: it takes the measurement points around the plot and says whether the ground is stable (within 2 millimetres a year), sinking slowly (between 2 and 5) or markedly (5 or more), with the trend from 2020 to 2024. It describes the area, not the individual building: cracks should be looked at by a qualified professional. Method in Ground movement.