Shufflecake is a tool for Linux that allows to create multiple hidden volumes on a storage device in such a way that it is very difficult, even under forensic inspection, to prove the existence of such volumes. This is useful for people whose freedom of expression is threatened by repressive authorities or dangerous criminal organizations, in particular: whistleblowers, investigative journalists, and activists for human rights in oppressive regimes. You can consider Shufflecake a “spiritual successor” of tools such as TrueCrypt and VeraCrypt, but vastly improved: it works natively on Linux, it supports any filesystem of choice, and can manage multiple nested volumes per device, so to make deniability of the existence of these partitions really plausible.
Check their FAQ too: Shufflecake