The Legend of Zelda/Underworld/Quest 2/Dungeon 1

Objectives

A middle section of this dungeon is optional. This walkthrough will only cover the rooms necessary to clear the level.
The entrance to this dungeon is found in the same place as the first dungeon in the first quest.
From the entrance and enter the right room. Defeat the Goriyas here to obtain the wooden boomerang.
Bomb the top wall to reveal a passage. Go through the passage and defeat the Keeses. When the room is cleared, push the left most block to gain access to the underground passage.
Take the passage and you will arrive towards the top of the dungeon. Defeat the Wall Masters here and obtain a key to unlock the left door.
Travel left and defeat all of the blue Goriyas. Bomb the top wall to expose a path to the boss of the dungeon.

The boss is Aquamentus. You must stike him in the head six times with the Wooden Sword or three times with the White Sword. Time your strikes between the energy orbs that he emits from his horn. Only the Magic Shield will absorb the energy orbs. Link will receive damage if he tries to block them with the regular shield.
Defeat him to earn a heart container. Enter the room to the right to obtain the first Triforce piece.