Just like a source of sound bouncing off nearby surfaces before reaching your ears, photons from a source near a black hole will have multiple echo’s due to the wrenching gravity.  What is truly cool about this discovery is it gives scientists a way to very accurately measure the mass of a black hole.  NASA scientists are presenting their findings this week.  Read more here:http://www.nasa.gov/centers/goddard/news/topstory/2008/blackhole_echo.html