Undercity Catacombs

Undercity Catacombs is a dungeon located in Tanren, in the Undercity. The only HMs needed to access it are the ones necessary for reaching the Undercity: Rock Smash and Rock Climb. Rock-, Ghost-, and Poison-type Pokémon are abundant here.

Lower Floors
The walls consist of brown stone walls with a darker brown stone floor. Water is found infrequently here, mostly in clumps of one or two tiles.
 * Floors 1-20

Upper Floors
The scenery stays the same as earlier floors. Water no longer spawns. Rooms become larger than previous floors.
 * Floors 21-30

Pokémon
The Pokémon here are Ground, Ghost, Dark, Steel, Rock-type.

Lower Floors
 

Boss

 * Onix (2)


 * Steelix

Ending Box

 * Ekop
 * Relic Copper
 * Relic Silver
 * Relic Gold
 * TM Payback
 * King's Rock
 * Psyche Dust
 * Reaper Cloth
 * Invisify Orb
 * Max Revive
 * Purple Bow
 * Restore Power
 * Rotten Apple
 * Metal Coat
 * Shiny Stone
 * Dawn Stone
 * Purple Gummi
 * Revive
 * Old Gateau

Secret Room
There are 2 boxes in the secret room. The boxes may contain the following items:
 * Nothing
 * Moon Stone
 * Sun Stone
 * Dusk Stone

(The list is incomplete, so feel free to edit it.)
 * Relic Vase
 * Lum Berry
 * Lunar Ribbon
 * Relic Copper
 * TM Shadow Ball
 * Max Revive
 * Revival Herb
 * Rare Fossil
 * Max Potion
 * Rare Bone
 * Relic Gold
 * Revive
 * Restore Power

Spawned Pokemon :

Additionally, using honey in the secret room has a chance of spawning recruitable Vullaby along with a horde of Mandibuzz.

Tips
2-3-7 is the fastest route through the puzzle on floor 20