MEXICO CITY - It turns out a partial solution to this unwieldy megacity's vexing water problem may have been under residents' feet -- albeit a long way down -- all along.

Mexico City government officials announced the discovery of an aquifer more than a mile beneath the earth's surface that could provide enough water for at least some of the metropolitan area's 20 million residents. Officials say the aquifer could reduce the city's dependence on water pumped from outlying areas and reduce the strain on the region's shallower aquifers -- the over-pumping of which is causing the city to sink precipitously, in some cases more than a foot each year.

The city's water department drilled an exploratory well recently in the eastern borough of Iztapalapa, a densely packed urban zone where the quality of water -- much of which is drawn from shallower depths -- is poor enough to be the punch line for many local jokes.

The news of a new water source was received with excitement in a city where experts have been predicting that demand for water could eventually outstrip supply. During a severe drought in 2009, the city government hauled water into some areas by truck.

In a front-page article Monday in the newspaper Reforma bearing the fevered headline "Water in Sight!" the director of the city water system, Ramon Aguirre, described the discovery as "one of the biggest historical successes for the city" capable of supplying water for more than 100 years.

In a subsequent radio interview, however, Aguirre emphasized that there was still much to be done, in concert with the federal government, to confirm the size of the aquifer and determine how much water could be extracted from it.

Aguirre said he expected the city to initially drill five wells to draw water from the aquifer, a project that could cost about $40 million.

Federico Mooser, a veteran geologist who helped the city with its exploratory well in Iztapalapa, emphasized in a phone interview that the aquifer would not solve Mexico City's water problem. "This is a lucky finding of medium importance," he said.

Still, Mooser said, it was a promising new source of water that, while not drinkable, could be made so with a relatively inexpensive treatment process. Just as importantly, he said, the aquifer is deep enough that tapping it would not cause subsidence.

"That's very important," he said. "Because with every ... well that we have in Mexico City, we take [water] out, and we sink."

Despite the discovery of the aquifer, part of the metropolitan area's long-term water solution will have to include the construction of more water treatment plants to recycle used water.