A Black Hole: A Dam

Never thought of it in this way, but in Tess Taylor's review of Caleb Scharf's Gravity's Engines, the case is made that a Black a dam:

On earth, in a dam, there is pressure on one side and a lack of pressure on the other. Water forced through spillways, driven by gravity, generates enormous energy that we harvest as electricity. In a black hole, there is the universe on one side and a void on the other. And as stars and particles rush towards black holes, they pick up speed, sloshing in much the same way water does heading towards a dam or drain. Just as sloshing water represents lost energy we hear converted to gurgling sound waves, stars and gasses rushing towards the brink of a cosmic drain lose particles that can be "seen" translated into other forms of energy. The edges of black holes are thus always spewing matter, a kind of cosmic splatter paint. Although things pulled towards black holes are mostly swallowed, over time the sloshing of nearly swallowed stars spews the universe with a mess of Jackson Pollock-like cosmic goo.