In the video, the telemetry says -49.5 feet after the crash. Thats NEGATIVE 49.5 feet. It must have been very high to drill itself so far underground.
Quadcopters rarely carry radar altimeters, so the altimeter data comes from one of two possible sources:
1) barometric pressure.
The former requires correction for current air temperature and pressure, a process that pilots accomplish by setting a reported value in their altimeter and verifying the displayed altitude against known airport elevation. Quad pilots not operating at an airport with a reported "altimeter setting" are likely to make any adjustment by setting current altitude to "0" (AGL). If you start flying on a hilltop, then any flight that goes below that hilltop will become negative. If you fail to set the pressure altitude to 0 before taking off, you can easily have a negative elevation for the entire time of the flight. The quad flight management will create its own "AGL" by subtracting the starting altitude, but since the pilot may want to know MSL based on a calibrated pressure it won't convert the recorded telemetry.
If it is a GPS elevation, you should know that most GPS 3D data reports neither "MSL" nor "AGL" but "EHT" -- ellipsoid height. That is, the elevation above a reference ellipsoid that approximates the surface of the earth but does not equal it. It is quite possible to have an EHT of -49.5 feet and still be above ground level.