The HL Universe is the main universe where the Half-Life series is set, starting with the original Half-Life (1998).
City 17 Street originally existed in this universe up until Gordon Freeman blew up the dark fusion reactor in an attempt to stop Wallace Breen. Nathan Johnson found himself thrown back in time, and in an attempt to prevent Breen's death and the destruction of City 17, he delayed Gordon Freeman's arrival in City 17. Although a seemingly minor change, it created problems in time and space. Unknown to Nathan, The Elders created a new parallel universe, which despite being very similar, does have major changes.