Also, nobody calls it the "mainframe". It's the "host environment". It took me a couple of weeks on my current project to work out why my new colleagues were so reluctant to modify the customer re-contact batch process - "oh, that's on the host environment!".
As someone whose first job out of uni almost 20 years ago was in a mainframe environment, I have no desire whatsoever to go back to it and would strongly recommend against working in COBOL if you can make a living using tech that was developed in the last 40 years.