Obsidian Lock Box

The box is only half the loot. Every hellstone and obsidian crate hands you one, and inside is a single item pulled from the underworld locked chest pool. That pool holds the best pre-hardmode gear in the game, which is the practical reason to fish in lava at all.

The catch is that you cannot open one without a Shadow Key sitting in your inventory, and that key comes out of the dungeon. Lava fishing before you have dealt with Skeletron just builds a stack of boxes you have to sit on. The key is never consumed, so a single one covers every box you will ever pull. Do not vendor a sealed box either. Opening it and selling whatever falls out always pays better.