Mincom, a Brisbane based tech company started in the late 80s with a suite for mining, and oil/gas production. At the time, they had 1-10 customers who paid a LOT of money. I am sure they are bigger now, but the fundamentals here are the same: you have to maintain almost every version of product back to the origin, and backport any change, because you can guarantee there is a mine in Kazakstan making a very large amount of profit, which is unwilling to upgrade, but is willing to pay you to maintain the legacy codebase.
TL;DR -If you support mining, expect to support the FORTRAN code you shipped them in 1960s.
Indeed. That was the very company I was thinking of! The Mincom EAM product (Ellipse) now sits with Hitachi and their technical mining products found their way to Datamine after ABB acquired Mincom, then divested the acquired assets a few years later. The resources sector has a gazillion lines of FORTRAN, some of it freshly coded (new codebase) within the last 10 years, believe it or not.