Get within five blocks of the very bottom of the map. To do this, reach the Underworld then find a deep-reaching Underworld tower. Dig through the floor into the ash until you reach the bottom while avoiding pools of lava. To make this easy, try to have the following.

Golden Armor or better
Nightmare Pickaxe or better (for mining through Obsidian)
Obsidian Skull (for protection from Hellstone)
Obsidian Skin Potion or Lava Charm (for protection against Lava)