The computer is a MicroVAX 3100 from the legendary Digital Equipment Corp. (DEC), and sports what at the time of its purchase in 1989 probably seemed like an awesome 12MB of RAM. It's running OpenVMS, which, it's interesting to note, was in use in 1989 and is still being produced today by Hewlett-Packard, though you'd have a hard time installing the latest version on that old machine. (Compaq bought DEC in 1998, and was in turn bought by HP in 2002, which is how HP came to own the rights to the OS.)
As for YouCantOutrunABear's machine, it's running a number of non-mission-critical programs, including some computations on silver price conversions; it also prints labels onto an almost-as-antiquated printer. It all could be done just as well with current equipment, but as is appropriate in a mining environment, YouCantOutrunABear says that "our 'things that need replacing' list goes in order of most to least deadly."
The ghost of a PDP-11
The PDP-11 is another classic machine from DEC, originating in the 1970s and being sold into the early '90s. One of the most common uses for the computer was for real-time process control and factory automation; and one of the longest-lived examples of such use was in a particularly quirky application in the U.S. Navy.
In the 1970s, the Navy and American Airlines built a machine called the Multistation Spatial Disorientation Device (MSDD). It was something like a souped-up version of the Scrambler, that ride you know and love from state fairs, where you're rotated in nesting cycles within cycles, creating either great joy or intense nausea, depending on how delicate your constitution is. The MSDD had the added feature of keeping its riders in the dark; the point was to simulate the sense of disorientation you'd feel in a plane at night or in a cloud bank, and demonstrate that in such situations your brain's approximation of what was happening around you bore little or no resemblance to actual physical reality.
The MSDD wasn't operated by a carnie, but rather by a skilled technician and a PDP-11 -- at least up until 2007, when the Navy finally admitted that it was getting too expensive and difficult to keep that venerable computer in action. Migration Specialties, a company specializing in computer migration, as you might guess from its name, helped the Navy move to a more modern machine -- but one that was designed to simulate a PDP-11 down to the last detail. All that Fortran code is still running on a PDP emulation card, which in turn is helping emulate a fighter jet spinning out of control in a dark night.
The flat-file database that wouldn't die
If you think getting things to change in the military's infrastructure is bad, try the true black hole of U.S. government IT: the IRS. For decades, all of the IRS's tax records were held in something called Individual Master File, an enormous flat file containing millions upon millions of records, stored on big spinning wheels of magnetic tape and accessed via Cobol code. This file was the cutting edge of tech sophistication in the 1960s when it was first implemented, but 50 years later it had grown a little rough around the edges, no?