The entrance is a Dark Hole in the south western end of Lumbridge Swamp.
Requirements to enter:
Light Source (at least a Candle, but a Bullseye Lantern is highly recommended)
Recommended items:
Spiny Helmet
Antipoison Potions
Food
Weapon
Armour
Extra items may be needed depending on what you are going to fight.
(See Slayer for items needed for certain enemies)
You may end up in this dungeon if you 'fall through the floor' while doing the Lost Tribe Quest. Just head north and squeeze through the pipe to get back into the caverns under Lumbridge Castle.
Notes:
There is a Candle Seller near the entrance. He sells candles for 1000gp each. He will also give you info about making Lanterns.
While in the dungeon, if light source goes out don't move. You will be attacked by a swarm of tiny biting insects which can causing lots of damage real fast with little hits.
The light blue dots on the map below are Wall Beasts (level 49). They reach out of holes in the wall when you walk past. If you are wearing your Spiny Helmet it will help to repel them so you take less damage when attacked.
Tears of Guthix Mini-Game Info:
The entrance to the Tears of Guthix Mini-Game is located in the south eastern end of the dungeon, with the Giant Frogs (level 99). You must complete the Tears of Guthix Quest to play. Playing this Mini Game will add experience to your lowest skill. You can play this game ONCE a week, once you play you have to wait 6 days before doing it again. You must also gain 1 Quest Point or 100000 total exp before you can play again.
To play the game talk to Juna, tell her a story. She will give you your bowl and let you into the cave for a short time. The walls of the cave have blue and green streams of tears running down them. Blue streams add to the amount tears to your bowl and green takes them away. Click on the blue streams of tears to collect them. Be careful not to collect from the green streams. When your time is up, you will go back to Juna and drink the tears gaining experience in your lowest skill.