Bluefin-21 drone and it's daunting task

Yesterday, the Australian Joint Agency Coordination Centre (JACC) sent the drone, autonomous underwater vehicle (AUV) or mini-submarine, Bluefin-21, to scour the seabed of southern Indian Ocean for the missing MH370. Scarcely 6 hours later, the drone resurfaced.
     The drone takes two hours to descend to the seabed and two hours to ascend to the surface. Each time it operates for 24 hours. If it reaches the depth of 4500 meter, it automatically resurfaces
     The head of JACC Angus Houston describes the work using Bluefin-21 as a slow and painstaking process. We carry on with the daunting task.