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A Mud Block is a soft Block that is similar to Dirt and 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 (60 feet in small worlds, 120 feet on large worlds), you can create an Underground Jungle. Mushroom Grass Seeds can also grow at above 0ft.
At Hardmode any Corrupted block will turn regular mud blocks that are up to three squares away into dirt. If there was Jungle Grass then it also will be converted to Corrupt Grass. This does not affect mushroom grass tiles. Even so, mud is converted very slowly and the resulting dirt cannot be converted further if grass cannot grow on it, making it a possible buffer to buy time against the encroaching corruption. To offset this, Mud Blocks can be crafted from Dirt Blocks, allowing players to create a new supply.