Mud Block

A Mud Block is a Dirt-like Block that can be found Underground and crafted near Water while having a Dirt Block in the inventory. Mushroom Grass Seeds and Jungle Grass Seeds can only grow on mud.

When placing a Jungle Grass Seed on a Mud Block at below a certain depth (700ft in small worlds, 1,000 feet on large worlds), you can create an Underground Jungle. Mushroom Grass Seeds will only grow at below 0ft.

The crafting recipe for dirt was added in patch 1.1, because the Corruption in hardmode can very slowly turn Mud Blocks into Dirt Blocks and therefore spread through, making it unobtainable after a long while.