BGL
From FSDeveloper Wiki
BGL is the file format used by Microsoft Flight Simulator to store scenery. It uses the .bgl file extension. BGL stands for BAO Graphical Language. Where BAO stands for Bruce Artwick Organisation, the original creator of the Flight Simulator series.
There are a lot of different BGL file, each with their own structure. For example terrain BGL files made with Resample, airport BGL files made with BGLComp or legacy BGL files made with BGLC or SCASM.