NASA says satellite photos show Saudi Arabia has been busy digging for a resource possibly more precious than oil -- water for agriculture.

The country has tapped hidden reserves of water to grow wheat and other crops in the Syrian Desert for 24 years -- a phenomenon visible in a time series of images acquired by three different Landsat satellites operated by NASA and the U.S. Geological Survey, the agency reported Thursday.

The green fields dotting the desert in the images draw on water that in part was trapped during the last Ice Age, and the "fossil" water is now in aquifers buried deep under the desert's shifting sands.

Water for directly irrigating the fields is reached in the underground rivers and lakes by drilling through the desert floor.

Because rainfall in the area is only about an inch a year, the underground water is a non-renewable resource, and hydrologists estimate it will only be economical to pump water for about 50 years, NASA said. (c) UPI