SquashFS
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported.
The Live CD versions of various Linux distributions mount SquashFS as the root filesystem and mostly use OverlayFS to provide a writable layer over SquashFS.
In Alpine Linux, the modloop file is a squashfs image. The modloop contains kernel modules and essential firmware files. During bootup, the modloop gets mounted in read only mode as a loop device at /.modloop.