Sewer

Sewers are the gimmicks of Steam Ages. They are holes found on the lawn. They either make toxic steam or help transport zombies. Flattened Mushroom can block these, but with some restrictions.

Variants of sewers
Sewers that give off steam first appear on Day 1. They start being covered by manholes, being harmless. However, later they fling their manhole out of the lawn to give off steam in a 3x3 area, damaging plants continuously. A Flattened Mushroom planted on it will stop the steam from coming out, along with it not getting damaged. This can only last a limited time as the steam will eventually fling the Flattened Mushroom out.

Sewers that transport zombies act much like Lost Guide Zombie's secret doors, but zombies are transported more slowly. Only small-to-medium sized zombies can by transported, while Gargantuars and machines ignore them. Flattened Mushroom can stop these from working, but if placed in an exit and zombies have taken entrance, they will eat the Flattened Mushroom. Flattened Mushrooms placed on entrances will make all zombies ignore entrances.