Salt water laced with cancer-causing chemicals, a byproduct of oil and gas drilling, is spewing from old wells. Experts warn of a pollution crisis spreading underground and threatening Oklahoma’s drinking water.

When oil and gas are pumped from the ground, they come up with briny fluid called “produced water,” many times saltier than the sea and laden with chemicals, including some that cause cancer. Most of this toxic water is shot back underground using what are known as injection wells.

Wastewater injection had been happening in Oklahoma for 80 years, but something was driving the growing number of purges. Ray and his colleagues in the oil division set out to find the cause. As they scoured well records and years of data, they zeroed in on a significant clue: The purges were occurring near wells where companies were injecting oil field wastewater at excessively high pressure, high enough to crack rock deep underground and allow the waste to travel uncontrolled for miles.