1Sniffing Out Bombs When the bombs went off at the Boston Marathon, highly trained dogs were rushed to the scene to search for more explosives. Boston police have said dogs swept the streets in the morning and a second time just an hour before the first marathoners crossed the finish line. It is considered likely that the bombers planted their devices well after the dogs finished sweeping the area. Since 9/11, dogs have been used more than ever because nothing has proven more effective against hidden bombs than the nose of a working dog. The best of them serve with United States Special Operations and they are in a league of their own. It is nearly impossible to get anyone to talk about them publicly because much of what they do is classified, but 20/20 was able to talk to the people who train them for this story. A look at what might have happened in Boston while getting a rare glimpse inside the secretive world of America's most elite dogs.

2Settling Scores This is a story about facing one's demons head on. In 2009, Navy clearance diver Paul de Gelder lost his arm and leg in a bull shark attack in Sydney Harbour. His zest for life and determination to succeed despite his horrific injuries inspired viewers. He was seemingly impervious. But Paul has been harboring a secret. He still has one score to settle, to go face to face with his nemesis - the bull shark.

3Marfa, Texas It sometimes seems that America is a country hopelessly divided -- by class, by politics, by culture. Tonight, 20/20 once again goes to a remote place where few have trodden before -- Marfa, Texas -- for a lesson in artful coexistence. Marfa is in cattle country, the high desert of far West Texas. Like many small towns, it has come close to extinction. But today, Marfa lives on, is even thriving: its renaissance spurred by the arrival of a host of young, cutting edge artists. Mixing cowboys and culture might seem like a bad idea, but it has made Marfa a capital of quirkiness.