but research on the exxon valdez spill released in september 2015 shows that even trace levels of crude oil can be toxic to fish embryos and may have contributed to collapses in salmon and hering populations in the prince william sound. another option is letting nature deal with the oil through biodegradation. >> biodegradation, in other words, having organisms that naturally occur in the water degrade the oil. it's a relatively slow process in the cold temperatures that we see in the arctic. that also can be a very slow and long term process. it's all a tradeoff if you have oil accumulating at the surface... you have birds, you have marine mammals that can come up thru that oil and that can be very problematic. the question is what are the impacts of those dispersants, how efficient are they, what's going to happen potentially to public health and to seafood safety, all of those questions. >> noaa scientists aboard the healy are also testing devices to help responders track oil, like this surveillance balloon called an aerostat. todd jacobs is the head of noaa's unmanned aircraft progr

because kaczynski mistakenly believed he had helped exxon clean up its public image after the exxon valdez just over three months later, the unabomber's reign of terror was suddenly overshadowed by a much more destructive blast. >> on the morning of april 19th we all get the call there's been a terrible bombing in oklahoma city. so we're really focused on this at this point, because everybody's going to be asking is this the unabomber. and we gave our best assessment then that we didn't think it was. because these are different personalities. these are different types of bombings. one is specific targeted, the other is a mass murder. >> any thought the unabomber was responsible for oklahoma city was quickly erased when timothy mcveigh was arrested two days after the blast. ted kaczynski, it seems, had his own agenda and his own timetable. >> when the oklahoma city bombing was happening, theodore kaczynski was already on a bus on his way to deliver the package for sending to his next victim. >> on april 24th, just five days after oklahoma city, a mail bomb killed gilbert murray. president o