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Cerulean Cave is kino as hell. Blocked off by an NPC because "you're not strong enough" to keep your interest. Inside there's a bunch of items and strong Pokemon for you to find once you've beaten the game, and clearing it rewards you with an OP (at least in gen 1) and overleveled mon.
I love how XY tries to replicate it only to remove the dungeon part of "post game dungeon with a legendary" making it boring and soulless as a result.