An ERS image of the Dead Sea...

The European Space Agency (Esa) Earth Resources Satellites (ERS-1 and ERS-2) surveyed the Dead Sea between 1992 and 1999 and identified regions that were sinking.

Subsidence rates have now been measured accurately for the first time. Typically, the land is going down across the whole region by two centimetres (less than one inch) a year, but data show some areas are falling by up to six centimetres a year.

During the observation period, the level of the Dead Sea itself fell by about six metres - part of a longer trend that has now been detected in the surrounding rocks.

Dead and dropping

The Dead Sea region contains the lowest point of dry land on the planet, and is flanked on two sides by sharply rising terrain - the Moab range to the west and the Edom range to the east.

...with the southern end enlarged

The lake is the saltiest body of water in the world. Situated at the mouth of the Jordan River, the Dead Sea has no outlet and forms part of the border between Jordan and Israel.

The Arabs call it Bahr Lut - the Sea of Lot. Several smaller streams pour into the sea from the east, bringing a heavy influx of fresh water, which evaporates due to the region's extreme heat. The high saltiness kills animal and plant life; fish carried in by the Jordan soon die.

During the 20th Century, the water level of the Dead Sea dropped continuously from about 390 metres (1,280 feet) below sea level in 1930 to 414 m (1,360 ft) below sea level in 1999, with the average rate of fall accelerating in recent years.

Relief map

It is known that the surrounding terrain is also subsiding, but putting reliable figures to this trend has required the use of precise satellite-based positioning techniques.

The ERS mission took measurments over the Dead Sea between 1992 and 1999

Researchers used the Interferometric Synthetic Aperture Radar (InSAR) facilities on the ERS satellites to detect the lowering of ground level in the region.

Two radar scans of the same region were taken at different times; changes in height were used to construct a two-dimensional map of crustal deformation.

The scientists, from the Geological Survey of Israel and the Scripps Institution of Oceanography in the US, say that as water drains away from the Dead Sea, the porous rocks beneath the region are drying out. It means the rocks are no longer able to support the weight of those above them and subside under the pressure.