Pizza Sewers is the second secret area implemented into the game. The enemies here are not too threatening and should be no trouble. Health pickups in this area look like pizza slices. They will drop from enemies and destructible Pizza Box props.
This area occurs between 2-1 and 3-1. The game treats it like 3-1, which means that the Weapon Chest in this area drops weapons from 3-1 and below. After exiting Pizza Sewers, the game's difficulty is increased by one. The regular 3-1 area becomes equivalent to 3-2 in difficulty, which means there are more enemies and weapons start dropping earlier.
How to enter the Pizza Sewers
In order to access the Pizza Sewers you must find a round manhole cover in the Sewers that bears a resemblance to a turtle's shell (sometimes it's hidden under a chest). This is the entrance to the Pizza Sewers. You can get in by blowing it up with an explosive weapon that causes regular explosions, such as a Grenade Launcher. Rogue's active and passive abilities will also work. Weapons that create Blood Explosions and Melting's active ability won't work. Blowing up the manhole opens a portal and at the same time kills almost every enemy in the Sewers, which can be used to your advantage as you don't have to enter the portal immediately. While looping the game, you can blow up the manhole only after killing the Mom boss.