The drilling mud used to lubricate the equipment was a clay called bentonite with small amounts of a polymer additive, according to Facebook. It’s biodegradable and nontoxic, the company tells The Verge. Bentonite on its own isn’t usually bad for the environment, John Dilles, another professor of geology at Oregon State, tells The Verge in an email. It’s sometimes used to absorb toxic metal at sites that need to be cleaned up or where waste needs to be contained, he says.<p>In the case of an accidental release, however, the drilling fluid components used at the site should be prevented from “entering sewers, waterways, or low areas,” according to safety data sheets published by the state. Facebook opted to leave 6,500 gallons of mud in place to “minimize the risk of leaks,” it wrote.<p>Facebook also left behind a drill tip, tools, and 1,100 feet of pipe. It’s not uncommon for companies to leave behind pieces of broken equipment when digging it up is more costly or could cause even more destruction. There’s a risk of losing more equipment when working through hard rock, according to Goldfinger.<p>Not much to discuss here.