Dungeon Location Investigation (5e Variant Rule)
From D&D Wiki
Dungeon Location Investigation
This is a fairly basic system meant to add a little more realism to the discovery of dungeons or special locations, or allow more opportunity for the DM to add flavour around dungeons.
Instead of having an NPC in the world just flat out say "There's a dungeon to the northwest!" or whathaveyou, or directly having players stumble on it, instead hint to the general location for a place the NPC doesn't know much about. (Why would a barkeep know where an old crypt is?) After dropping this hint, push the players to move to the area to investigate. Then, have them each roll a Perception check, DC 15. If 2 or more players (assuming a group of 5 players, raise or lower number at your discretion) succeed the check, the players have found the location. If less than the threshold succeed, the party must spend time looking for clues in the area for the dungeon. Minor dungeons require 2 clues, major 3, and plot-relevant dungeons 5. Each clue requires a DC10 Investigation check, and only 1 player needs to succeed the check to get the clue.
The party must travel between areas to look for clues, with at least 1 mile of travel to look in new places after exhausting previous ones. After 1 day, the party may return to previously searched areas to look again.
Optional Variant: +/- Clues for Crits
If any character rolls a natural 20 when investigating for clues, they may receive 2 clues from one check.
If any character rolls a natural 1 when investigating for clues, the party loses 1 clue.
Optional Variant: Dungeons for Clues
You can draw out the main campaign by hiding clues to the final dungeon within other dungeons, requiring the clearance of the minor dungeons first. Putting the final dungeon in a place one wouldn't be able to find without clues (In the sky, under the sea, underground, etc.) makes this make more sense; it'd be a bit silly if the final area was just next door to the starting area, but still somehow makes the players run all over the world.