Deep Underground - Ancient Cross Country Travel Tunnels

Ug had an idea for new contents. Inspirational reading helps.
Blogs used to do it for me. Exhausted on youtube vids

Working on the last project on my list for a product.
Possibly escaping from this will spur other interesting ideas.

This will fit in with other projects featuring underground exodus and exploration.
Dungeoneers survival guide was a favourite of mine that helped me dnd a few more years. Veins of the Earth and Downcrawl are handy books. Operation Unfathomable handy too. A version of this for ancient sub tunnels possible but on a bigger scale than what I have done before.  

http://elfmaidsandoctopi.blogspot.com/2018/01/d100-wilderness-caves.html
https://elfmaidsandoctopi.blogspot.com/2019/12/d100-ancient-caves-of-third-age.html
http://elfmaidsandoctopi.blogspot.com/2017/12/colony-quest-in-underland.html
http://elfmaidsandoctopi.blogspot.com/2017/12/colony-quest-in-underland-2-exploring.html
http://elfmaidsandoctopi.blogspot.com/2017/12/colony-quest-in-underland-3-d100-great.html
http://elfmaidsandoctopi.blogspot.com/2017/09/subsurface-exploration.html
http://elfmaidsandoctopi.blogspot.com/2014/11/mushroomland-encounters.html
https://elfmaidsandoctopi.blogspot.com/2017/09/underland-citadel.html (map)
http://elfmaidsandoctopi.blogspot.com/2017/01/sinister-underland-of-east.html

Possibly encounter tables for various depths of the earth
1 - subsurface realms where surface and underlanders meet
2 - the underland realms where kingdoms trade and war 
3 - the deep dark depths of ancient eldritch evil and banished horror
4- borders of underworld and hells entry gates in a great cavern under the earth where the world tree has its roots gnawed on by dragons and hydra

Deep Underground - Ancient Cross Country Travel Tunnels

In ancient times the Underland and the surface world mixed more often. Subsequent apocalypses and sea rising ended this age but great underground highways remain crossing seas, mountains and even the great nations of humanity. Mostly they are forgotten but demihumans, humanoids and a few humans know the secrets. 

The humans who still know are often nomad minorities, secretive cults and rangers who mysteriously travel these secret but potentially deadly secret highways. They tend to be between ancient rivers as the builders used waterways separately. Sea levels changed and rivers moved so this is not always true. The underhighway often connected ports and great crossings. Those rare humans know certain routes but not the deeper safer ones where the underland folk trade connecting great chasm regions and strange folk. In ancient times peoples fled great calamities via these passages, some remained in the Underland, others came to the surface. Many areas are lost to monsters, not just goblins or kobolds but the worst horrors and eaters of humanity. 

Intact great underhighway of the deep link underland regions and kingdoms

Mark several of the main routes across your campaign map. It's ok to connect islands or sites of sunken cities and the most ancient ruins and places where underland races dwell. 

d12 Tunnels Dark Deep
1 Thin narrow crawlways, many natural caves following underground streams
2 Sprawling irregular tunnels once mined following veins of metal left by bleeding gods
3 Tunnels dug by ancient huge creatures possibly directed by intelligence
4 Crude megalithic tunnels with supports and tunnels through rock
5 Magnificent naturals wide cave tunnels with bridges over water and obstacles

6 Great chasm crack with water features and fungus forests, often very high ceilings

7 Bored by some great apparatus used long ago with variegated textured walls

8 Lava tubes of jagged black basalt and cooled lava formations

9 Great wide dressed brick-lined tunnels made for trade by kobolds or demihumans

10 Cyclopean black stones with symbols of chaos and darkness made in age of darkness

11 Smooth wide tunnels melted with wizardry, even and glassy

12 Dug by a god or giants, wide and high, with great stone supports in places 

d12 Condition & Usability
1 Crawling with forgotten horrors and evils of the elder darkness, areas sealed

2 Forgotten, badly damaged and broken into segments by blockages

3 Worn and damaged with some blockages, only used by desperate

4 Seldom used by anyone and some areas damaged or about to collapse

5 Damaged by constant warfare between intelligent species 

6 Several secretive peoples use and make repairs but leave some hazards for unwary

7 Travellers us and occasionally repair and provide warnings of hazards

8 A colony of humanoids or demihumans maintain and use for their daily business

9 Used by several underland kingdoms who maintain parts and operate checkpoints

10 Used by several civilisations who maintain it and have toll stations

11 Maintained by secretive beings for aeons

12 Maintained by ancient magics automatically

d12 Blocked Tunnels
1 Cave-in caused a partial blockage
2 Huge cave-in and a change in orientation
3 Barricaded by someone and forgotten
4 Colony of humanoids or demihumans
5 Flooded by waters
6 Poison gas, smoke or lava
7 Colony of monsters
8 Hidden under fungus forest
9 Monster filled eco system

10 Humanoid or demihuman gatehouse, fort or toll station
11 Planar gate to elemental worlds or roots of the world tree
12 Planar gate to lower planes, underworld or hell

d12 Side Passages (Per Day)

1 Surface air vent d4 1=hidden hole 2=rocky shaft 3=cave 4=well

2 Surface passage d4 1=tiny crack 2=secret door 3=hidden gate 4=dungeon

3 Cavern complex d6 1=dead 2=ruins 3=thriving ecosystem 4=colony 5=art 6=burial

4 Ancient mines d4 1=ancient mine 2=quarry 3=cursed mine 4=occupied dig

5 Ancient ruins d4 1=prehistoric city 2=megalithic site 3=ruined tower 4=ruined fort 

6 Holyground d4 1=graveyard 2=megalithic tombs 3=ruined temple 4=ruined fort 

7 Settlement d4 1=fortified house 2=tiny shack 3=village 4=outpost

8 Colony of humanoids or demihumans who control a surface gateway

9 Dead ends d4 1=monster lair 2=humanoid camp 3=outpost 4=bodies 

10 Junction to another highway system

11 Decent to the deeper highway network

12 Strange Cavern with some wondrous sight 

d12 Common Caverns
1 Barren but beautiful cave 1in6 chance of water, 1in6 chance of a lair

2 Lake chasm with islands 1in6 with ruins, 1in6 chance of a lair

3 Fungus gardens and forest rich in cave life

4 Ruins of ancients, an outpost or colony abandoned or destroyed by a disaster

5 Home of a loner or a group of exiles living in seclusion

6 Outpost of humanoid or demihuman kingdom

7 Colony of humanoid or demihuman kin to a local kingdom

8 Cavern is a battleground with resources that kingdoms fued over

9 Estate of some being with a servitor race and a mansion or palace or castle  

10 Area with water, fish and fungus where traders meet in peace

11 Remains of diggings d4 1=quarry 2=mine 3=ruined temple 4=forbidden crypts

12 Roll on rare caverns or prehistoric art and megaliths

d12 Rare Caverns

1 Artificial light from a great ruined citadel at the centre of the dead cavern

2  Artificial light from great thriving citadel at the centre of a populated cavern

3 Giant crystals, exotic minerals and strange earth creatures

4 A fiery lake of lava, ash heaps and smoke. Lava and fire creatures dwell here

5 Lake and water feature (d4 1=island 2=waterfall 3=whirlpoor 4=sunken ruins) populated by aquatic creatures

6 Great windy cavern with flying and air creatures 

7 Ice cavern with frozen tunnel ridden glacier and creatures of cold and ice

8 Artificial light from cavern roof, grows strange fern forests and albino cave dinosaurs

9 Imprisoned ancient evil elder in the vault with occult warnings 

10 Artificial light on a great ruined citadel at the centre of the dead cavern

11 Great redoubt where some beings fled to preserve themselves from some apocalypse,  now a colony or asleep by some means

12 Remains of some ancient god or titanic kaiju, some attract cult pilgrims, wizard looters or treasure hunters. Petty gods or guardian spirits may protect such sites 

d12 Common Underland Folk

1 Dwarf 

2 Gnome

3 Elf

4 Halfling

5 Goblinoid - d4 1=goblins 2=hobgoblins 3=bugbears 4=

6 Orc - often made by wizards during ancient wars now free and hate wizards

7 Kobold - tiny goblinoids with many sub-breeds and types

8 Reptilian folk - d6 1=lizardfolk 2=iguanafolk 3=pteradactylfolk 4=snakefolk 5=turtlefolk 6=dinofolk

9 Animal abhumans - d4 1=batfolk 2=molefolk 3=lampreyfolk 4=fishfolk

10 Animal abhumans - d4 1=crabfolk 2=bugfolk 3=lampreyfolk 4=fishfolk

11 Mushroom folk  - d4 1=shroomlings 2=toadstool folk 3=mushroom folk 4=shroom giants

12 Giantkin - d4 1=ogres 2=trolls 3=cyclops 4=cavefolk or roll on Rare Underland Folk

d12 Rare Underland Folk

1 Giants d4 1=cave 2=stone 3=lava 4=formarian

2 Undead d4 1=skeletons 2=zombies 3=shadows 4=spectral minions 5=ghouls 6=mummies 7=wights 4=undead giants or other monsters

3 Earth abhumans d4 1=ironfolk 2=crystalfolk 3=rockfolk 4=clayfolk

4 Other elemental abhumans d4 1=mistfolk 2=icefolk 3=waterfolk 4=magmafolk

5 Rare Goblinoids d4 1=norkers 2=darkstalkers 3=gremlin 4=nilbogs

6 Underground feyfolk d4 1=mushroom fairy 2=cave nymph 3=cave hag 4=dark elves 

7 Evil spawn of the deep d4 1=doppelgangers 2=troglodytes 3=mongrelfolk 4=ogre magi 

8 Amphibians folk d4 1=newtfolk 2=frogfolk 3=toadfolk 4=olmfolk 

9 Elder race of the depths d4 1=amonitefolk 2=trilobitefolk 3=scorpionfolk 4=spiderfolk

10 Elder horror race d4 1= squidfolk mind mages 2=eldritch slugfolk 3=elder newts 4=catfishfolk sorcerers 

11 Rare unique race, a remnant of the past, unlikely to ever see species twice

12 Unique doomed rare race, only tribe left in the world

d12 Gates & Seals

1 Sealed by tons of rock rubble or a single huge stone block

2 Huge fort gate barred on inside weighs ton to move

3 Huge complex trapped mechanical lock

4 Huge magically sealed gate

5 Hive of some monsters blocked opening

6 Magic illusion covers the opening

7 Gate only visible under certain stars or full moon
8 Barricaded with rubble and improvised tracks

9 Bricked up long ago by isolationists

10 After a massacre gate haunted by undead

11 A dragon has moved in and charges toll to pass

12 Devils converted to a gate to hell, often devils guard now