The project around 10 million lines of code. Some of it was written in a very specific version of Fortran that was a PITA to compile. One fun experience was opening a file and seeing it was created on my birthday. Not in the sense just matching the day and month of my birtdhay. The code was literally written on the day I was born.
I recall rewriting a several thousand line program that was written in a preprocessing language that compiled to fortran that was homerolled by some random guy that was just so ahead of the game at the time that he basically wrote an extension of fortran with all the macros he made and rolled into one thing.
