in 2010 purcell and his crew searched with no luck. another attempt, with three vehicles around the clock, took more than a week before they found what seemed to be the plane. >> it's fun to be on a little hunt out there on the water to try to find something. >> it gets your pulse going a little bit. >> they sent the sub back underwater to take the photographs that confirmed it. >> at what point does it become practical. >> it becomes practical once they get more information that would narrow the search area. >> the search area in the air france case was specific - about 5,500. investigators had a clue within days where the planent down. >> purcell says any debris in the ocean currently could be pushed by the ocean. >> the search area on the surface is huge. it's not practical to do that kind of search with underwater vehicles. it would take the rest of my lifetime and part of the my next one to conduct the search. >> even if the safe area shrinks soon, purcell says the process of underwater searching will take a while. it could be mon

so they called in mike purcell and his team. the same team that famously found the titanic. not only did purcell's team find the black boxes in the wreckage, after two years in the water they still worked. today in woods hole purcell told us about the similarities and the differences between the searches for the two airplanes. >> there are some similarities. people want to know why it happened. i think that the location of this crash is a little bit different from the air france one. so once the underwater search starts i think that it will be pretty easy to find it. >> reporter: the final investigation report concluded that the mechanical error was mishandled and misunderstood by the pilots, who in turn caused the plane to crash by their actions. some lessons for this investigation, as details begin to emerge. but first they must answer the biggest mystery so far. where is the airplane? for "nightline" i'm elizabeth vargas in new york. >> mm. some eerie similarities. although the two incidents did involve different types of planes as well as different weather conditions. >>

mike purcell is the principal engineer here. >> they can go up and down mountains that are up to 40 degreesin slope. they are very stable so you get really good data almost all the time. >> reporter: why would this underwater robot find something even the u.s. navy and search teams for from more than two dozen different countries have been been able to find? first of all, the torpedo shaped vehicles can reach depths of more than three and a half miles below the surface. and it can survey wide swathes of the ocean floor using what's called side scan sonar. >> they send a sound pulse that's sort of a fan beam out to the side and it will travel out almost half a mile from the vehicle and it bounces off the sea floor and we get a reflection back to the vehicle. >> reporter: they call the process mowing the lawn because it works its assigned grid back and forth before returning to the surface with images captured on a high-resolution camera. it's all done at the touch of a laptop on dry land. how do you tell the difference? how do you know if it's a fish or rock or plane? >> you can tell from th

mike purcell is the principle engineer here. >> they can go up and down mountains that are up to 40 degrees in slope. they are very stable, so you get really good data almost all of the time. >> reporter: why would this underwater robot find something even the u.s. navy and search teams from more than two dozen different countries haven't been able to find? first of all, the torpedo-shaped vehicle can reach depths of up to 6,000 meters or more than three-and-a-half miles below the surface. and it can survey water swaths of the ocean floor using what's called side scan s.o.n.a.r. >> they send a sound pulse that's sort of a fan beam out to the side, and it will it travel out almost half a mile from the vehicle. and it bounces off the sea floor, and we get a reflection back to the vehicle. >> reporter: they call the process mowing the lawn, because it works its assigned grid back and forth before returning to the surface with images. captured on a high-resolution camera. it's all done at the touch of a laptop on dry land. >> how do you tell the difference? how would you know if it's a fish, ro

well exactly what you're saying purcell as for the ultimatum the ad the russian defense ministry and this is now according to russian media sources of calling it utter nonsense you know the year the regional talk of this ultimatum was i a russian news agency report. interfax cited someone close to the ukrainian defense ministry. it was hearsay based on hearsay based on hearsay. you have to take everything very prudently in a story like this on a lot of the russian news agencies in particular have the most to you you almost feel like you can use to tyneside as sounding boards to lead to float. sometimes you know rumors and theories. suppose the facts that turned out to be on refuted afterwards of what i can tell you is the situation the past couple of days here from where we are in the administrative capital of the crime yes in pro ball has been really extraordinary on and surreal at the same time. um it's not a classic coup d'etat in the sense of personal coup d'etat czar typically internal affairs states again states. this is it's basically green men in camouflage uniforms who as i

mike purcell is the principal engineer here. >> they can go up and down mountains that are up to 40 degreesvery stable so you get really good data almost all the time. >> reporter: why would this underwater robot find something even u.s. navy and search teams from more than two dozen different countries have been able to find? the torpedo-shaped vehicle can reach depths up to 6,000 meters, or more than 3 1/2 miles below the surface. and it can survey wide swaths of the ocean floor, using what's called side scan sonar. >> they send a sound pulse that's sort of a fan beam out to the side. it will travel out almost half a mile from the vehicle. and it bounces off the sea floor and we get a reflection back to the vehicle. >> reporter: they call the process mows the lawn because it works its assigned grid back and forth before returning to the surface with images, captured on a high resolution camera. it's all done at the search of a laptop on dry land. >> how do you tell the difference? how would you know if it's a fish or rock or plane engine? >> you can tell from the return. man made objects,

mike purcell is the principal engineer here. >> they can go up and down mountains that are up to 40 degrees in slope. they are very stable so you get really good data almost all the time. >> >> reporter: why would this robot find something that the u.s. navy and two other countries haven't been able to find? first of all, the torpedo vehicle can reach depths up to 6,000 meters or more than 3.5 miles below the surface and it can survey wide swathes of the ocean floor using what's called side scan sonar. >> they send a sound pulse that's sort of a fan beam out to the side. it will travel out almost half a mile from the vehicle and it bounces off the sea floor and we get a reflection back to the vehicle. >> reporter: they call the process mowing the lawn because it works its assigned grid back and forth before returning to the surface with images captured on a high resolution camera. it's all done at the touch of a laptop on dry land. >> reporter: how would you tell the difference? >> you can tell from return. manmade objects, metd tall down there on the sea floor responds very strongly. >> re