You come to a door that warns of a pit beyond. The door is locked; the key is held by a Dungeon Wanderer, so you must defeat a Dungeon Wanderer before you can open this door.
Do not open the door if you wish to explore the Dungeon further. Opening the door will lead to combat with the area boss, the Dungeon Master, and if you defeat the Dungeon Master the only action available to you will be to jump into the Pit...and find the game's second area, the Hollow Earth, at the bottom.
Note that there is a Pit Room Door Glitch.