ARG/Cistern Mask
Cistern Mask[1] is a post-release ARG set piece added to INFRA in November 11, 2019 update.
Setup
In chapter 7, Working Overtime, in Cistern map module, a makeshift homeless shelter, found at the top of West Stalburg Runoff Discharge System silo, contains a collectable key inside a metal can.
Continuing that map module, near the end, a locked gate door with the Stalburg Underground graffiti and a Keep Out! sign is found.
Unlocking the door with a key leads towards a boarded hallway, through the boards a unique Mask[2] is seen, and a disturbing audio resembling distorted speech is heard.
Puzzle
The extracted or recorded audio can be reversed to hear an intelligible speech. Throughout the audio two different voices are heard, one of them transcribes to:
Below the underground city, there is a hatch, where they sleep, the raven is red, but why? What is the connection?
The other voice appears to be speaking Finnish[3], part of which appears to be "metro tunneliin tehty kaupunki", which translates to "city built into the metro tunnels"[3]
Besides the two main voices, a background which appears to be generic public chatter is heard, and a repeating clicking noise is present. Audio spectogram reflects that and should be further investigated.