It is called a hog lagoon. The excrement and urine are collected into lined pits so it doesn't get in the ground water. It is a huge issue environmentally. Most states in the US, in the south at least, won't let you build any new hog lagoons due to the environmental impact.

If you've never smelled it, you can't describe it to someone. It is a worse smell than you can possibly imagine. Modern chicken coops are a close second. You can smell either of them a half a mile away in a car with your windows rolled up.