The Wishing Well/Sink Hole Entrance: The players enter a Well/Sink Hole and lower themselves by rope or other means and start in the pool cave.
The Underground River Entrance: The party travel along an underground river and enter the dungeon by climbing down the waterfalls.
The Cave/Sewer Entrance: The easy way!
The pool could have a hole in the bottom, lots of things could pop out to say hello to the party.
Rocks can fall at any time, especially if the party start throwing around loud spells. Have a few rocks fall from the roof to spice up any combat encounter. Make it clear to the party the walls are unstable before you bury them!
Necromancers are always looking for nice quiet places to raise the dead, this place is prime real estate!
Undead could start climbing out of the well at night.
Maybe the smugglers are all dead, eaten by some nasty creature, or maybe they have become undead themselves!
The room to the right can be accessed by crawling up the rock slide from the main crypt corridor. However its quite a tight fit… and maybe there are Zombies buried there, which should be fun for whomever is trying to squeeze through.
The other entrance to the room on the right is a steep 10 foot climb, crumbling dirt and rocks make it quite a hard task to climb, and also noisy.