25 Underground Plot Hooks for D&D and Fantasy TTRPGs, Photo Sketch of an Underground Tunnel

25 Underground Plot Hooks

Want to send your players into the depths of the earth? Need a side-quest idea for your cavern-based campaign?

Role Player’s Respite has put together a list of underground-based plot hooks for you to use in your D&D or other TTRPG campaign! These plot hooks are free to use at your table. All I ask is that you leave a comment below letting me know how it went in your game.

Make sure to check out more of my D&D and TTRPG plot hooks to use in your game!

Without further ado, here are 25 plot hooks for underground adventures.

25 Underground Plot Hooks for D&D & Other Fantasy TTRPGs

  1. Miners discover a seemingly endless cave system. The tunnels take them further and further beneath the surface before they come across strange creatures that force them back. Further exploration shows that the tunnels venture to impossible depths and continue without the end in sight.
  2. The monarch calls on adventurers to delve into the depths of a recently discovered cave. Supposedly, the cave leads to the tomb of the first ruler who kept a staff that only those of royal blood may use.
  3. An underground city wants to reestablish routes connecting to other cities using an ancient, underground roadway. Over the centuries, the roads have become overrun with monsters and paths blocked by rubble. As such, the city’s council puts out the call for anyone willing to assist in clearing the roadways for repairs and construction.
  4. People around the world are vanishing with the only trace of their disappearance being a hole in the ground. Every hole is large enough for the missing person to fit through. Many nations call upon adventurers to investigate the holes to find any clues as to what’s happening.
  5. Several underground crystals provide life-sustaining energy to the land above. Something starts corrupting the crystals, causing the land to wither and rot. Stewards of the crystals call for help from various nations to restore the crystals and stop whatever is happening.
  6. A malevolent group of deep-cave dwellers are somehow swallowing entire town into enormous sink holes. The affected towns are growing in size, indicating that the group is developing their way up to sinking entire cities at a time.
  7. An eccentric wizard claims to have hidden a magical treasure at the bottom of an impossibly deep cave system. They’ve stocked the caves with increasingly powerful monsters further down.
  8. An underground war breaks out between various cave dwellers. Each faction vies for supremacy over the others, part of which involves seeking to hire adventurers as mercenaries of sorts.
  9. An earth elemental cult seeks to start the Great Expansion, a process that eventually causes the earth to expand beyond its normal size, engulfing the surface. They’ve started rituals that have caused new mountain ranges to form, swallowing towns and killing hundreds. But, they won’t be satisfied until the entirety of the world’s current surface is buried.
  10. Ancient underground sigils seal a great wyrm in an eternal slumber. Recently, unusual earthquakes have plagued the surface, leading from scholars to believe someone or something is interfering with the sigils in an attempt to awaken the wyrm.
  11. An organization dedicated to keeping a malevolent force deep underground seeks aid in their duties as attempted incursions by their sworn enemy increase in intensity and number.
  12. Scholars discover a massive, sprawling, living underground structure. Tunnels weave throughout the underground system that terminate in "organs" that facilitate the creature. These scholars speculate that the entity descends deep beneath the earth, but exploring it proves dangerous as many creatures have taken up residence withint the entity.
  13. A massive and ancient underground city sprawls beneath a major city and surrounding area. Scholars, nobles, and would-be adventurers explore the ruins in search of lost artifacts, but encounter strange and horrific creatures stalking the streets.
  14. An apocalyptic, magical calamity looms over the world. Many nations turn to digging and establishing underground safe havens, but conflict with creatures of the caves beneath the earth cause difficulties in developing these shelters.
  15. A deep underground calamity spills over onto the surface, cracking the surface and altering environments in unexpected ways. Many of the dwellers of the deep flee to the surface, desperately seeking aid in stopping whatever is causing this event.
  16. A mage accidentally releases several rare and violent monsters into a vast underground compound. They hire adventurers to find and either capture or eliminate the creatures before they make their way to the surface.
  17. The giant moles excavation and mining crews use go on a rampage. Several of them begin tunneling great distances, creating vast networks of intertwining caves and causing sinkholes across the region.
  18. A mining company shuts down and abandons all of their operations. Many of the miners wanted management to give a proper send off to the small fey creatures that helped in their daily duties, but this was written off as superstition. Now, every town near one of the newly abandoned mines experience odd tremors, goods going missing, and minor injuries that progressively get more and more serious.
  19. Fire elementals begin carving tunnels beneath various settlements, towns, and cities. These tunnels all connect at a nexus designed to eventually erupt in a massive volcanic explosion originating from the plane of fire. Researchers and scholars call upon leaders of various communities to find willing individuals to enter the tunnels and stop the elementals from completing their structure.
  20. A cult of earth-elemental worshippers are working towards pulling settlements into the vast caverns of the Elemental Plane of Earth to serve wicked dao masters. Some settlements have already fallen prey to their workings, and they continue to open ever-growing, subterranean portals to the earth plane, releasing increasingly dangerous earth elementals.
  21. The prisoners of an underground prison stage a mass riot and escape. Now, they roam the various tunnel systems and cave networks connected to the prison. Surface officials hire bounty hunters to delve the depths to find the escaped criminals.
  22. A dwarven city’s miners dig ever deeper to find rarer and rarer materials. Unfortunately for them, they dug too deep and awakened an ancient, malevolent entity that now seeks to destroy the city.
  23. A large mineshaft collapses, trapping several dozen miners throughout the underground network. People on the surface do what they can to help, seeking anyone brave enough to venture into the tunnels to save anyone they can.
  24. Large worms plague various settlements and farmsteads. It’s gotten to the point where the farmers and some town officials seek hiring anyone willing to seek the source of these worms and put an end to them.
  25. The Professional Spelunking Association, an organization dedicated to the creation and hosting of manufactured dungeon delving events, starts their annual Tournament of Adventurous Delving. Adventurers, mages, mercenaries, private militaries, and many other individuals and groups register to compete for the grand prize. The team to beat the hardest dungeon first, wins!

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