Quake’s E1M7 — ‘The House of Chthon’ — contains a secret level entrance leading to E1M8, ‘Ziggurat Vertigo,’ a level set in a void above a floating platform where the player fights on a structure that moves and tilts. This secret level is accessible only by finding a button hidden beneath the lava at the base of the main level — requiring a deliberate leap into what appears to be instant death.
The level design of Quake blends medieval architecture, science fiction portals, and Lovecraftian entities in a way that id Software never fully explained as intentional. John Romero has confirmed that the aesthetic collision was deliberate — a shotgun wedding between the fantasy setting the team had been working with in DOOM’s expansion campaigns and the alien horror they wanted for the sequel. The result is an aesthetic that belongs to neither genre.

Didn’t realise how much was hidden under the surface. Makes me want to replay it with fresh eyes.
This is why I always check every corner. You never know what’s been left for the observant player.