Septic

The Septic is one of the environments in Chapter 3. It is unlocked at the beginning of the game. The theme of the Septic is poison and ipecac. The player can only access Septic after Caves 1.

In the Septic floor treasure rooms spawn 3 items, with 2 of the items being an item included in Genesis+ rather than being chosen from the treasure room pool (though if re-rolled the new item will be chosen from treasure pool), this was done to reward players for going to septic (similar to 4 items in the starting room of 'the chest') and also to make it easier for people to try out the new items.

How To Enter
You can enter Septic by going through the poisonous gate that appears when destroying a tinted rock in the middle of a room in The Caves Chapter 2 (this can be in a secret room so you might miss it), once broken you will find a small sewer hole that can be unlocked with a key.

The Sewer Hole that goes to Septic.

Music
The music for Septic is titled Venenum. Venenum is latin for Poison. Thus, Venenum literally means "Poison". Listen here