A major focus of the debate over hydraulic fracturing, or "fracking" to get oil and gas out of the ground is the stuff put IN the ground to get it out.
The specifics of what's in the fracking fluid are not always communicated or required.
Canadian journalist Andrew Nikiforuk found a long history of fracking and a long list of items put into the ground, which he reported in his book Slick Water from 2015. We replay the interview here.