Secret

Secrets are special areas in Quake that are not accessible by straightforward means. Secrets contain valuable items such as Ammo, Weapons, Powerups, or even Level Exits to unique maps called "Secret Levels". Secrets can be identified by dissimilarities that disrupt patterns in a level. For example, in E1M1: The Slipgate Complex, there exists a row of lights illuminating identical textures on the right side of the first corridor, but it is unusual as one of the textures is different due to being illuminated by a blinking light instead of a constant one. Hitting the dissimilar texture will cause it to open, revealing a cache of Shells. A message will appear and a sound effect will play every time a Secret has been found. Every level has a counter for the amount of Secrets that have been discovered by somebody.

Secrets are inherited from id Software’s previous titles; notably, Doom and the earlier Wolfenstein 3D. New features such as freelook and jumping lead to new ideas for Secrets.

Index
In total, Quake has 113 Secrets.

Episode 1: Dimension of the Doomed

 * E1M1: The Slipgate Complex: 6
 * E1M2: Castle of the Damned: 3
 * E1M3: The Necropolis: 3
 * E1M4: The Grisly Grotto: 3
 * E1M5: Gloom Keep: 5
 * E1M6: The Door to Chthon: 4
 * E1M7: The House of Chthon: 0
 * E1M8: Ziggurat Vertigo (secret level): 2

Episode 2: The Realm of Black Magic

 * E2M1: The Installation: 7
 * E2M2: The Ogre Citadel: 3
 * E2M3: The Crypt of Decay: 4
 * E2M4: The Ebon Fortress: 4
 * E2M5: The Wizard's Manse: 2
 * E2M6: The Dismal Oubliette: 2
 * E2M7: The Underearth (secret level): 2

Episode 3: The Netherworld

 * E3M1: Termination Central: 5
 * E3M2: The Vaults of Zin: 3
 * E3M3: The Tomb of Terror: 2
 * E3M4: Satan's Dark Delight: 4
 * E3M5: The Wind Tunnels: 4
 * E3M6: Chambers of Torment: 2
 * E3M7: The Haunted Halls (secret level): 4

Episode 4: The Elder World

 * E4M1: The Sewage System: 4
 * E4M2: The Tower of Despair: 5
 * E4M3: The Elder God Shrine: 3
 * E4M4: the Palace of Hate: 5
 * E4M5: Hell's Atrium: 4
 * E4M6: The Pain Maze: 4
 * E4M7: Azure Agony: 9
 * E4M8: The Nameless City (secret level): 4

Final Level

 * Shub-Niggurath's Pit: 1