Tar Bubble
Tar Bubbles are stationary obstacles that don't deal contact damage, and constantly emits a puddle of slowing black creep around it. Walking over a bubble will destroy it, creating either an x or + shaped puddle of black creep. If destroyed by any other means, such as being shot at by Isaac's tears, it releases a shower of harmful black projectiles nearby instead.
Tar Bubbles do not need to be destroyed to clear the room, and will harmlessly disappear if no other enemies are remaining.
Tar Bubbles are primarily spawned by various 
Dank Depths enemies, but can also appear on their own in some room layouts.
Variants
Spider Egg
Spider Eggs are variants of Tar Bubbles that emit a pool of slowing white creep instead. If walked over, it will be destroyed and create either an x or + shaped puddle of white creep. If destroyed by any other means, it releases a swarm of 
Baby Spiders instead.
Spider Eggs, like Tar Bubbles, do not need to be destroyed to clear the room, and will harmlessly disappear if no other enemies are remaining.
Spider Eggs are primarily spawned by 
Spinneretches, but can also appear on their own in some room layouts.
