As engine dependability has greatly increased, many twin-engine planes have become able to carry out over-ocean flights. Search and rescue on the open ocean is very difficult, with high labor requirements and costs. However, in order to determine the causes of accidents and prevent their re-occurrence, recovery of the plane's black box and any remains in the ocean is necessary. It is only natural that the public's desire for a safe society calls for the thorough investigation of accidents through searches at sea.