What is "physicsfs"

PhysicsFS (PhysFS for short) is a zlib-licensed programming library written in C for virtual file access. PhysicsFS is designed to provide a hardware abstraction layer to the developer, allowing them to access files contained within various archive formats as well as direct access to the filesystem, which would all be represented in a single virtual Unix-like file tree. PhysicsFS initially supported ZIP, Quake PAK, and Doom WAD file formats, but recent editions expand filetype support to LZMA-compressed 7z, Build engine GRP, and Descent HOG and MVL formats.