Half-Life (1998)

Half-Life features no classic flying saucers. Its alien threat arrives not by spacecraft but through a physics experiment gone wrong: playing a theoretical physicist deep inside the remote Black Mesa Research Facility, you handle an anomalous crystal and trigger a chain "resonance cascade" that tears open teleport rifts to the borderworld of Xen, through which the creatures pour in. The game therefore belongs to the "interdimensional invasion" tradition rather than the nuts-and-bolts "visiting spaceship" one—echoing a real minority view in ufology that blames anomalies on cross-dimensional entities rather than star travelers^[1]. Told entirely in first person with almost no cutscenes, it rewrote the shooter and is widely ranked among the most influential games ever made^[2].