As the drought continues, Arizona may have to rely more on groundwater and less on the Colorado River, but groundwater pumping may have consequences.

In 2007, a fissure in Chandler Heights swallowed a 13-year-old horse named Cash. The fissures can be a byproduct of groundwater pumping. Although some aquifers are made of solid rock, others are less stable.

They’re made of gravel and sand, held in place by the pressure of groundwater. When that water is removed, the geology rearranges itself, creating fissures and sinkholes, said Grace Carlson, a post-graduate researcher at Arizona State University.

“Some of them are quite small, but they can get to be really, really large, like more than a kilometer long, four meters wide, 30 meters deep, so they can be really dangerous," Carlson said. 

Attempts to fill in the fissures have been unsuccessful; once one has opened, there is not much that can be done about it.