The Ram Sandstone is a large transboundary aquifer of Cambro-Ordovician age, shared between Jordan and Saudi Arabia. It is 500 to more than 4000 m thick and stores fossil groundwater 10 000–35 000 years old. Modern recharge is probably insignificant and the aquifer has been in a state of depletion since at least the last humid interludes of 5000–10 000 years ago. Despite its long residence, the groundwater is generally fresh. Recently concentrations of radium exceeding accepted maximum limits have been identified. Exploitation in Jordan has been mainly in the Southern Desert, where groundwater levels and well depths are relatively modest. In the past, irrigation was the predominant use with smaller amounts supplying the coastal city of Aqaba. A scheme to supply the capital Amman with 100 million cubic metres per year for 50 years was completed in 2013. Being a fossil aquifer, its utilization will reduce the stock available for future generations, but with prudent planning, extractions should be sustainable and depletion may not reach a critical level. However, the Dead Sea thermal springs that have been enjoyed since historical times and the baseflow of deeply incised valleys could be affected.