The term 'SPELUNKER's milieu' refers to the environment of a SPELUNKER, which is someone who explores CAVES. The crossword solutions for this term are 'CAVES' (5 letters) and 'CAVE' (4 letters). CAVES are natural underground formations that can be found in many different types of terrain, from mountains to deserts. They provide a unique environment for exploration, with their own unique ecosystems and geological features. Spelunkers explore these CAVES, often using specialized equipment and techniques to navigate them safely.