Bottom of the Well

In Ocarina of Time, Kakariko Village was home to a well. That well was the entrance to a Dungeon, which contained an underground chamber, in which the sacred Lens of Truth was hidden. When Link became an adult, the well was sealed, and when he was a child, it was temporarily filled with water. In order to enter the dungeon, young Link had to play the Song of Storms to super-charge the windmill and drain the well dry.

The well contains many illusionary walls and floors which could only be identified as fake using Lens of Truth, while in other places floors were invisible requiring the Lens of Truth once again to reveal such illusions. Deep within the Bottom of the Well, Link fights one of the freakiest foes imaginable, Dead Hand, which will yield the Lens of Truth upon death. However, it is possible to skip the majority of the well, and acquire the Lens of Truth in less than 10 minutes, making the only alternate reason anybody would want to go through the whole thing is to pick up some Gold Skulltula Tokens and a Piece of Heart.

On the whole, this dungeon is completely optional, having no effect on the plot or completion if not attempted. However, without the Lens of Truth, not only will the last two temples and Ganon's Castle be of immense difficulty, you may miss out on many hidden Heart Pieces and treasures only the Lens of Truth can uncover throughout Hyrule itself.

Minor Enemies and Traps
ReDeads  Gibdos  Green Bubbles  Like Likes  Beamos  Wallmasters  Skulltulas  Gold Skulltulas  As well as invisible floors, fake walls, trap floors, and poisonous water.

Trivia

 * The house of a man who could see the truth once stood on the location of the well.
 * A signpost near the well's entrance reads "Dark! Narrow! Scary! Well of Three Features"