When an earthquake struck Oklahoma on Saturday, one of the first steps state officials took was to shut down 37 of the state's 3,200 active disposal wells -- a move that drew national attention to the link between oil and gas drilling and earthquakes.
No one was seriously injured in the Oklahoma quake, and investigations and cleanup has begun. Here's what you need to know.
What's a disposal well?
Disposal wells are used by oil and gas producers to get rid of wastewater from the drilling process. The wells push the wastewater deep underground, even deeper than where oil and gas are found.
The wastewater mostly consists of a substance called brine -- a mix of water and chemicals that comes to the surface with oil and gas when they are pumped from the Earth.
A small portion is also the water that's pumped underground in the modern hydraulic fracturing process, a drilling technique often referred to as fracking.
The EPA says there are about 40,000 disposal wells nationwide.
Did the disposal wells cause the earthquake?
That is the concern. Oklahoma didn't have much of a history of earthquakes. But a big one struck in November 2011, causing injuries and leveling houses, and officials said in 2014 that earthquakes in the state have increased 5,000%.
Scientists began looking into the role pumping liquid underground can play in seismic activity decades ago. And while much remains unknown, government scientists said in March that disposal wells have caused an increased risk for "induced" earthquakes in large areas of the country.
That includes the area where Saturday's earthquake occurred.
What's being done to stop it?
Oklahoma regulators have been monitoring the wells for years. In January, the state began forcing 27 disposal wells to reduce their activity. That's also when Oklahoma dedicated $1.4 million to fund "research and response" efforts.
Saturday's order that 37 wells shut down over the next 10 days marks the strongest action against disposal wells yet.
Why are disposal wells used?
The Oklahoma Oil & Gas Association said in April that producers would take "significant measures" to combat the earthquakes and promised to explore alternative disposal methods.
But OKOGA president Chad Warmington said "underground wastewater disposal is currently the safest and most cost-effective way to dispose of produced water."
Drillers also argue that recycling is more expensive, in part because they must pay to transport the wastewater to recycling facilities.
Some drillers do recycle. For example, about 90% of wastewater in Pennsylvania is recycled. In Texas, drought has spurred an increase in recycling.
But recycling has been slow to catch on in Oklahoma, and regulators have been criticized for not taking more aggressive action against disposal wells.
What happens next?
That's not clear. The state of Oklahoma has declared a state of emergency in the wake of Saturday's quake.
Halting or slowing the use of disposal wells won't necessarily resolve the seismic issues. Scientists studying the issue told PBS in March that even if Oklahoma shuttered all its disposal wells, the wastewater that has already been pumped can continue to cause problems.
And the ground is sensitive to sudden changes in disposal well pumping. In fact, Oklahoma regulators ordered the 37 wells offline over the course of 10 days for fear "that a large scale, sudden shutdown could cause an earthquake," according to authorities.