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

Objectives

The entrance to this dungeon is easily found in the bottom left corner of section 1. Approach it from the area to the south.
From the entrance, pick up one key from the room immediately to the right, and pick up another key in the room above and to the left of the entrance.
From the entrance, go up once and right twice, to find the compass. The north wall of the compass room can be bombed to reveal a hidden passage.
Above the compass room, you can obtain the map. The north wall of the map room can also be bombed.
Above the map room, defeat the three Goriyas to obtain the magic boomerang. Be careful of the energy orbs fired from the statues in the four corners. The north wall can be bombed.
From the magic boomerang room, the left room and the room above that one both contain keys. The room above the magic boomerang room has bombs. The north wall of that room can also be bombed.
Three rooms above the boomerang room, a man gives you a hint about how to defeat the boss of the dungeon. The left wall of the man's room can be bombed.
From the man's room, go left one room, and up one room to find the boss.

The boss is Dodongo. There are two ways to beat him. Either place two bombs in front of Dodongo so that he eats them, or place a bomb so that it blows up in his face before he eats it. If it blows up in his face, he will be stunned and you can stab him and earn another set of bombs.
Defeat him to earn a heart container. Continue on to the left room to obtain the second piece of the Triforce.