S.T.A.L.K.E.R. CoC Atmosphere

Call of Chernobyl is a fan-made standalone mod for S.T.A.L.K.E.R. built on the X-Ray engine, released around 2016. It stitches all three original games' maps into one open-world freeplay sandbox. The audio leans heavily on the original games' atmospheric design: wind across abandoned villages, distant anomalies, and the periodic Geiger ticking that defines the entire series.

I've put more hours into CoC than into any official S.T.A.L.K.E.R. game, mostly because freeplay mode strips out the story pressure and just lets you wander. The Yanov-Kopachy ambient is one of the most distinctive zones in the mod. Useful for productivity if you can tolerate the occasional far-off gunshot.