The Otherside is the dimension added by the Deeper and Darker mod, and it’s a dark cave world trapped between a bedrock floor and a bedrock ceiling, a lot like the Nether. You’ll get there through the portal in the middle of an Ancient City, once it’s been lit with a 
Heart of the Deep from the Warden.
It’s split into four biomes, each with its own stone, plants and mobs, and the Ancient Temple buried in the Deeplands is the only structure you’ll find there. Most of the mod’s materials come from the Otherside, like 
Resonarium, 
Sculk Bones and 
Soul Crystals, so you’ll be spending a lot of time down there.
Getting There
The only way in is the Otherside Portal, which is lit inside the 
Reinforced Deepslate frame at the center of an Ancient City. Clear any Sculk out of the opening, then use the heart on the inside of the frame, and the portal will fill the whole archway.
The Otherside uses the same coordinates as the Overworld, so walking 100 blocks down there moves you 100 blocks up here as well. When you arrive, the mod builds a return portal for you if there isn’t one within 128 blocks, usually inside a cave with enough room to stand up.
Terrain
The whole dimension is 128 blocks tall, with layers of bedrock at the top and bottom, and the space in between is carved out into huge caverns. The stone you’ll be digging through is 
Sculk Stone rather than Stone or Deepslate, and it’s swapped for 
Gloomslate in the Overcast Columns.
There’s no water level or lava sea down there either, as the lowest open spaces below Y 17 are filled in with 
Sculk Grime instead. It’s a soft block a lot like Mud, which you’ll sink into a little as you walk, so the bottom of most caverns is a flat floor of it.
It’s always midnight in the Otherside, and there’s no sky light at all, so the only light comes from glowing plants, blocks like 
Sculk Gleam, and whatever you bring with you. That makes it a lot darker than anything in the Overworld, even though there’s no Darkness effect from the dimension itself.
Biomes
The four biomes are spread across the whole dimension in large patches, and each one is easy to recognize from the ground it’s built on. The table below shows what makes each of them stand out, and every biome has its own page with the full details.
| Biome | Ground | What Stands Out |
|---|---|---|
| Deeplands | Sculk Stone and Sculk | Ancient Temples, columns of Sculk Stone, Sculk Gleam |
| Echoing Forest | Echo Soil | Echo Trees, drifting ash |
| Blooming Caverns | Blooming Sculk Stone and Blooming Moss | Pools of water, Glowing Roots and Vines, Bloom plants |
| Overcast Columns | Gloomslate and Gloomy Sculk | Gloomslate columns, Magma Blocks, Soul Sand and Soul Soil, smoke |
Exploring all four will earn you the Echolocation advancement, which is a good excuse to see each of them. The Deeplands is the one you’ll end up spending the most time in, though, as it’s the only biome where Ancient Temples generate.
Mobs
Every mob in the Otherside is part of the Sculk, and they spawn no matter how bright an area is, so lighting up a cave won’t keep it clear the way it does in the Overworld. Each biome has its own set of mobs, and the Overcast Columns doesn’t have any that spawn on their own.
| Biome | Mobs |
|---|---|
| Deeplands | Sculk Snapper, Sculk Centipede, Shattered, Phantom |
| Echoing Forest | Shattered, Sculk Snapper |
| Blooming Caverns | Sludge, Sculk Snapper, Angler Fish in the water |
| Overcast Columns | None |
A few more come out of the blocks around you rather than spawning normally. Shriek Worms burst out of 
Infested Sculk, Sculk Leeches hide inside 
Ancient Vases and 
Crystallized Amber, and the Stalker only appears when a fake vase is broken.
The Warden doesn’t spawn in the Otherside on its own, so the only way to meet one there is by setting off the Sculk Shriekers in an Ancient Temple. That’s worth remembering when you’re looting the temple, as a Warden in those narrow halls is a lot harder to escape.
Resources
All of the usual ores generate in the Otherside, from Coal and Iron right up to Diamonds and Emeralds, though they’re set in Sculk Stone or Gloomslate instead of Stone. Emeralds are a lot easier to find than in the Overworld, as they turn up in every biome rather than only in the mountains.
The 
Sculk Jaw is worth watching out for while you’re mining, as it’s a trap block that bites anything standing on it. Each biome also has its own building blocks, like 
Echo Logs in the Echoing Forest and the Gloomslate of the Overcast Columns, and there’s more on these on the Sculk Stone and Gloomslate and Echo and Bloom Wood pages.
What the Mobs Drop
The rarer materials come from the mobs rather than the ground, like Resonarium from Sludges, Sculk Bones from the Shattered and Soul Crystals from Stalkers. These go into the mod’s best gear, like Resonarium armor, the 
Soul Elytra and the 
Sonorous Staff, so it’s worth fighting the mobs instead of avoiding them.
Surviving
The biggest problem down there is how little you’re able to see, so it’s worth bringing plenty of torches, even though they won’t stop mobs spawning. The Sculk Shriekers in Ancient Temples can give you Darkness as well, and a 
Warden Helmet will clear both Darkness and Blindness for as long as you’re wearing it.
Beds work in the Otherside, so you’re able to sleep in one to set your spawn next to your portal. It won’t skip the night, since time never moves down there, and Respawn Anchors don’t work at all, so a Bed is the only way to set a spawn point.
Make sure to note down where your portal is as well, as every cave starts to look the same after a while. There’s no way to find your way back to it except by memory or by its coordinates, as the Ancient Compass only points to temples.
FAQ
With Deeper and Darker installed, kill the Warden in an Ancient City for a Heart of the Deep, then use it on the Reinforced Deepslate frame in the middle of the city. The frame will light up as an Otherside Portal, and walking into it will take you there.
The Warden doesn’t spawn there naturally, as none of the four biomes have it in their spawns. It can still be summoned by the Sculk Shriekers inside an Ancient Temple, though, just like the ones in an Ancient City.
Beds work in the Otherside, so you’re able to sleep in one and set your spawn point there. Time never moves in the dimension, which means sleeping won’t skip the night, and Respawn Anchors won’t work there at all.
Walk back through the portal you arrived in, as it links to the closest Otherside Portal at the same coordinates in the Overworld. If you’ve lost it, you’re able to light the frame in an Ancient Temple with another Heart of the Deep, or die to return to your spawn.
