⏚ Discover the mysteries of interconnected underground biomes. From vast ancient sewers that extend between villages to underground forests and seas, surprise might be waiting right beneath your feet ⏚

Undermod connects its underbiomes with one another in a meaningful way, creating new integrated exploration adventures in the underground. Explore dangerous places and find novel blocks to bring home for your builds!

Available for Minecraft 1.20 and 1.16, coming soon to newer versions. Forge for now. MC 1.12 and server-only versions are also on the horizon. Compatible with most world generation mods.

Created by Mystronics

*Most textures and models by GGVegan

What is Undermod?

• Adds natural and artificial underground biomes that depend on surface biomes. All features can be enabled/disabled in the configuration file.
• Adapts to existings worlds so you don't miss anything, preventing hard seams between old and new chunks.
• Connects faraway underbiomes with each other. For example, sewers usually discharge their water into underground rivers or seas.
• Completely procedural. No fixed designs! Undermod generation rewards exploration and keeps the door open for low-probability extreme cases (for example, megacaves). There is always the possibility of surprise.
• Works along with other world generation mods and follows the spirit of vanilla Minecraft.
• Powered by new Mystronics world generation engine, optimized for low lag.

- Underbiomes -

If Undermod gets enough downloads, you will see more of these coming!

Ancient Sewers

Sewers can be found under Minecraft villages, sometimes linking multiple villages together through a net of flooded underground tunnels that stretch beneath the land. Water falls through narrow channels into larger ones, eventually discharging into underground rivers, or occasionally subterranean seas. The air is thick and water tainted, be prepared for poison.

Wandering these passages, one might encounter long forgotten chambers - some abandoned, other repurposed. Outcasts have made these places their home - expect old living quarters, bandit hideouts, and more. Some say that villagers built on top of sewers for their hidden wealth and secret paths, while others believe that their ancestors took part in their construction.

Wildlife has also settled inside sewers. Mushrooms, spiders, rats and slimes are well adapted to this biome. And in the dark waters, something moves…

Tips to find them

Look for subtle entrances in villages. However, you will need to visit several villages to find sewers. Occasionally, they might be visible when they cross chasms or rivers. An easier way to visit sewers is by buying exploration maps from cartographer villagers. If you are out of emeralds or far from villages, it's also possible to find sewer maps hidden by bandits in underground forests.

Underground Forests

Surface forests can extend downwards in Undermod. Trees and other plants prosper under sinkholes, that let sunlight through into underground caverns. Certain type of small tree can even thrive under very dim light conditions, populating deeper caves that are completely dark to the human eye. This biome is very rich in coal, and can feature small amounts of lapis lazuli or even diamonds and glowstone. Given their size, underground forests usually give way to natural caves and chasms, and in rare cases explorers may also find connections to mineshafts, underground rivers or even ancient sewers, making these forests not just places of life, but crossroads in the depths.

Tips to find them

Underground forests are fairly common, so you should be able to find them by exploring surface forests. Search for pits or sinkholes that show you what lies below. However, not all surface forests have an underground counterpart.

Craters

Formed by the impact of fallen asteroids, craters were carved into to the Overworld by massive explosions. These depressions cut through the earth below sea level, exposing caves, chasms and new mineshafts to the sun. Remains of large meteorites can normally be found at their center, with diverse mineral composition. Over time, nature has reclaimed these craters. Depending on their location, they may be covered in grass and trees, filled with water, or left as barren wastelands. Explorers are always on the lookout for meteorites because they say they can make you a fortune. However, be it out of superstition or due to some deeper knowledge, villagers avoid settlements anywhere near these eerie places; unworldly scars of the earth…

Tips to find them

Craters are obviously easy to spot at surface level, but they are few and far between. If you want to find one without extensive exploration, your best bet is to look for old maps lost within ancient sewers, that mark areas of known celestial impact sites.

Underground Rivers

Water makes its way below the surface in Undermod, creating long rivers that can extend under the Overworld. Explorers find underground rivers specially useful, because they provide a mostly safe passage to move by boat between distant surface points across a region. They feature less minerals than normal caves, but can connect several cave systems together. Some rivers eventually flow into subterranean seas.

Tips to find them

It's very hard to find underground rivers by exploring the surface because they lie several blocks beneath. If you manage to reach a subterranean sea, investigate its contours in search for rivers - there is a good chance one or more rivers empty their water into it. A more reliable method is to follow the water: sewers usually drain into rivers - trace their flow downward from village to an outfall, where the sewer meets the river current.

Subterranean Seas

All underground water needs to go somewhere, and so, eventually underground river streams feed into subterranean seas. Some no larger than a lake, others so vast that they stretch beyond sight, creating underground horizons. Despite the absence of sunlight, life has found a way to survive here. Some fish and isolated leeches can be found deep in the water. Drowned zombies are a potential threat. Like in rivers, water has washed away most minerals over time, but subterranean seas are gateways to a myriad of caves, and potentially underground structures to explore for treasure.

Tips to find them

Subterranean seas are uncommon and lie deep below surface. Occasionally, chasms or even the surface ocean floor can show cracks that reveal their presence, but such openings are rare. Instead, a much easier way to find them is through underground rivers that flow into them. Also, old maps to subterranean seas have been lost inside Ancient Sewers…

Caverns

Undermod generates a variety of uncommon caverns of various sizes, making the underground (and mountains!) more interesting to explorers. You can recognize them by their stalactites. Larger cave volumes usually expose more minerals.

Tips to find them

Explorer maps to caverns are not a thing, but caverns are fairly frequent. Sometimes they break through to the surface, but more often they remain hidden beneath layers of stone. Usually you'll need some old fashioned underground exploration to stumble upon them. They are more common under plains.

- Mobs -

Leech

From blackened waters where light doesn't dare to shine, the leech is born. Unclean, as conjured from the filth itself, it drifts through murky waters always looking for its next victim, and for each stolen drop of life the leech restores its own vital energy. It's possible to use a forge to harness the life draining power from their teeth, but only piercing weapons can take it.

They avoid dry areas, as they are clumsy outside water, but never underestimate them when they swim, specially when they attack in numbers. The only thing they seem to fear is flaming fire; its heat driving them back into the depths.

Rat

Fast and quiet. Afraid of humans and cats, rats dash through damp sewer tunnels and small caves where predators can't reach them. They climb on walls and are able to slip through the bars of Understeel Fences. Either they are bloodless, or they have struck some unspoken pact with the leeches, for they can swim through the creatures’ water pools unnoticed.

Rats are usually harmless, but they are quick to snatch anything left unattended.

- Blocks -

Bring new blocks and ways of building from your adventures to your base!

Heaps of Filth

These piles of decay are magnets for certain types of creatures. When submerged under water, they slowly attract new leeches. On solid ground, they invite slimes. The spawning is slow, and can only happen when placed in murky biomes, like sewers or swamps. A single block is not sufficient - you need a decent amount to start the process. The larger the heap, the more likely it is to spawn these creatures.

Uses and Dangers

If you want to clear all leeches and slimes from your sewers, the only way to stop them from returning is to clean out every Heap of Filth. But filth isn’t always a burden - the experienced explorer might find its own uses for it. You could potentially move them somewhere else to function as a farm, but beware: moving these heaps around always results in some of the material being lost along the way.

Also, for those willing to keep digging through the grime, there’s always a chance of finding something shiny buried within, long forgotten inside the heap.

Understeel Blocks

Understeel is a versatile metal used for creating thin, grated structures. For now, the only way to get these blocks is by mining them in Ancient Sewers. Fences alone can be used to craft the rest of them, but not the other way around.

Understeel fences function similarly to glass panes but attach to the edges of blocks rather than the center, making them useful for building cages, railings, and barriers with a more open design. Understeel trapdoors and doors complete the set, giving more options for construction. Trapdoors cannot be opened manually but respond to redstone signals, making them ideal for grated floors, but also mechanical activation builds. Doors, on the contrary, work like traditional doors, serving as manual metal doors.

Damp bricks, understeel fences and small barrels; used to create a storage room on the walls of a sewers tunnel.

More

There are more secret items waiting to be discovered in Undermod.

Quick tips:

• Black Veil Mushrooms can only grow under very dark conditions, but if you can make your farm work, they are a wonderful alternative to Nether Wart for brewing.
• Lost Key and Occult Hand Mirror are still placeholders, so you won't be able to do anything with them yet. You can sell mirrors to high-level Clerics for 32 emeralds, but if you manage to find one of these items you might want to keep it - future versions of Undermod will reveal their purpose. They are very rare!

Come exploring!