so this is a well. >> claudia faunt: this is a well.ve a tape here that has a sensor on the end. >> stahl: oh, let me see. the geological survey has 20,000 wells like this across the country. it's a tape measure. >> faunt: it's a tape measure. >> stahl: how will you know when it's hit water? >> faunt: it's going to beep. >> stahl: by comparing measurements from different wells over time, they get the best picture they can of where groundwater levels stand. she unspooled and unspooled, until finally... ( beeping ) >> stahl: oh. it startled me, as did the result--- a five-foot drop in just one month. >> faunt: right now, we're reaching water levels that are at historic lows. they're like... >> stahl: historic lows? >> faunt: right. at this site, water levels have dropped about 200 feet in the last few years. >> stahl: gathering data from holes in the ground like this has been the only way to get a handle on groundwater depletion. that is, until 2002 and the launch of an experimental nasa satellite called grace. what does "grace" stand for