Wolfram Language / Mathematica. Probably not "underrated" since it is backed by an entire company with the sole purpose of developing it -- but as a programming

Wolfram Language / Mathematica. Probably not "underrated" since it is backed by an entire company with the sole purpose of developing it -- but as a programming language it is absolutely underappreciated. * Essentially a Lisp. Data and program can be freely interchanged and is frequently done in completely normal code. * The most impressive "standard" "library" in existence: everything from symbolic calculus, machine learning, UI building, graphics, signal processing, data import and export, foreign function interfaces and it just goes on and on. * Incredible attention to consistency and interoperability. Everything behaves in predictable and orthogonal fashions. Right from the low-level pattern matching up to interacting with the build in database. * The best documentation of any programming language. Nothing compares even slightly in breath, friendliness, consistency.