A black box, technically known as a Flight Recorder, is the main and the most important source of information in case of an airplane crash with no survivors.
The black box is generally placed at the tail of the aircraft, as the tail is the last part of the airplane to face impact in an accident, thus enabling it to record maximum information and face minimum damage.
The design of a black box is such that it can withstand an acceleration that is 3400 times the force of gravity, and an impact velocity of about 310 miles per hour. Additionally, it can also withstand extreme temperatures of up to 2000°F, and -67°F during a crash. These survival tests consist of the same or more impact that would be felt during an actual accident. A black box must be able to record about 25 hours of flight data.
A Flight Recorder can survive underwater, even if it is lost in the water body due to currents. Since the black box is equipped with an underwater locator beacon, it makes it easier for search parties to locate it in expansive waters. The beacon generates ‘pings’ at the speed of one ping per second, and keeps doing so for 30 days at a stretch. This gives search parties enough time to locate the black box, even if it is far away from the wreckage.