The Underrooms are the Backrooms but mirrored in a way. The Underrooms are as if your creepy basement had a creepier basement. Some of the levels in the Backrooms lead directly to them, a painting in Level 161e covers a hole in the wall that clips into Underrooms Level 0, clipping from Level 99 may result in teleporting to any Underrooms level.
The Level
The level is a very dim version of Backrooms Level 0, instead of walls there are mostly pillars, instead of a carpeted floor it is slightly worn concrete. There are some flipped over tables and even a few payphones on the walls. As for ambience, the constant sound of metal clanking, loud footsteps, thunder (possibly), and power generators malfunctioning and buzzing. Some lights may be broken or hanging from the ceiling by wires. The walls are a dry, worn down yellow wallpaper and the ceiling is tiled. The smell of almond water is spread across the entire level, and liquid pain is in broken vending machines. Any breaks in the walls clip right into Underrooms Level 1.
Entities
The level is home to Insanities, Skinstealers, Lighters, Hounds, and Death Moths, giving it danger level 4. There is a cult of normal humans inside, "The Afflicted," who worship the giant severed hand of an unknown beast, frequently sacrificing Insanities to it. Stalkers, while unconfirmed, are rumored to exist on this level. Finally, it has its own entities, The Mass is a boss entity which resides in a special parking lot-type room, made up of dead rats, assortments of all bugs, birds, and even a few human limbs, with two party-pooper bodies deep inside. It can punch down hard enough kill a human instantly, and uses a mass of tentacles to grab anything that runs away to assimilate it. It has minions, the Many, which are small black fluid liquids on the ground which tentacles and arms will sprout out of to grab entities and pull them in. Lastly, it has the poles, which are very tall and slim entities with insanely dexterous limbs with the pattern of the pillars around the facility as their skin and texture. When something gets close, it grabs it and slowly absorbs it, leaving behind a husk for a body.
Oddities
Oddities are any special attributes the level may have that are rare or completely exclusive to it, excluding entities. There is a speaker system in the level that will occasionally play 1930's-1950's jazz and romance songs without any static even if records of the music itself are distorted and static, they always play in perfect quality like you were watching the band in person. There is also small plant life living in the cracks of the concrete, mainly small flowers and some Venus Fly Traps which will eat the entity bugs that may approach them, they are also the source of the Almond Water smell (Maybe an ingredient?). It also is the first level of the very limited Underrooms and one of three that are accessible by normal levels or noclipping from reality. This level houses boxes of Mindcookies, which take on the appearance of cookies with small purple dots around them. Eating one will result in enhanced perception, awareness, charisma, and overall thinking logically, as well as making the person's sight brighter and letting them see previously unknown colors documented as "Inrex," "Bigala," and "Pheta,". Eating too many of these cookies may cause internal bleeding, abdominal pain, extreme stress, and eventually instantaneous combustion.
Entrances and Exits
Entrances are by noclipping through church stairways, noclipping through a hidden hole in Level 161e, and entering mentally via going insane in Levels 0, 1, 99, G64, and Level Fun. You can enter through Level 99 normally but it is unknown how exactly, possibly clipping.
As for exits, temporarily killing the Mass will transport you anywhere below 3 difficulty, eating too many Mindcookies will make you go insane and mentally exit to reality, or by finding an elevator somewhere that can lead to the Poolrooms or the Frontrooms.