I have noticed in several locations that when using the terrain display on the ND the minimum elevation is quite often "-13" (minus 13), which I believe would indicate -1,300 ft.
Here is an example:
Terrain Display - Minimum Elevation
Terrain Display - Minimum Elevation
Thierry (aka Captain Tango) | Website: https://www.flightpathsimulation.club | YouTube: https://www.youtube.com/@captaintango