Underhell is a Half Life 2 Modification developed by Mxthe in collaboration with We Create Stuff.
Underhell Prologue requires a Source Engine game to run, and Source SDK Base 2006 installed.
Underhell Chapter 1 will require a Source Engine game to run, and Source SDK Base 2007 installed.
Underhell Chapter 1 Trailer has arrived! It features a lot of voices from many different conversations throughout the game.
This should give you an idea of what to expect as far as ambient and immersion is concerned.
Mod DB Page : Desura.com
Website : Underhell.wecreatestuff.com
Steam Community Page : Steamcommunity.com