Mainframe / COBOL
Also written as Mainframe, COBOL, z/OS, IBM Z, CICS, JCL
The large IBM machines and the 1960s-era COBOL language that still process most bank card payments, insurance claims and government benefits. The systems are old but critical, and there are fewer and fewer people who know them.
Think of it like
The original plumbing in an old building: nobody wants to rip it out because everything depends on it, and few plumbers still know how it was laid.
Junior or senior?
Legacy on the adoption curve but highly differentiating on a resume. The talent pool is shrinking as people retire, and it is in demand for migration projects.
Senior sounds like
Has worked on moving mainframe workloads to modern systems, not only maintaining them.
Ask them
“Have you been involved in moving any mainframe work onto modern systems? What made it hard?”