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Subterranean

Source Galaxy Exploration Manual pg. 84
Between a planet’s core and its crust lie caverns lit with softly glowing crystals and fungi, underground rivers and seas, ancient stone fortresses, and metropolitan technological wonders. These are just a few of the myriad adventure locations one can find within a planet’s subterranean reaches.
When adventurers observe a planet from orbit, they might see verdant forests, vast deserts, jagged mountains, shimmering oceans, or sprawling metropolises. However, these environments merely scratch the surface of what adventures a planet might hold. As brave heroes traverse the depths, who knows what wonders they might find as they journey through the seemingly infinite network of tunnels and caves?
Subterranean environments exist parallel to the surface world, covering just as much, if not more, area as their aboveground counterparts. While explorers might find themselves in cramped tunnels and passageways, their underground adventures might also take them across vast open caverns or enable them to cross a planet via a network of caves. In a science fantasy adventure, the heroes can use all manner of technological equipment and magical methods to navigate the dark—darkness that can hide threats, challenges, and discoveries the likes of which surface dwellers have never seen. Adventurers might need to rappel down cliffs, scramble over craggy boulders, carve pathways into the rock, and navigate a wide range of other environmental challenges to discover ancient, long-forgotten treasures and ruins.
Subterranean environments are replete with bizarre life forms and discoveries that often differ greatly from those of the surface. Darkness is a defining feature of underground environments, and the flora and fauna that dwell within have likely adapted over the eons to life without a sun. Mosses and fungi might glow with bioluminescence or slowly crawl across surfaces in vast migrations. Animals might have surpassed the need for eyesight and use other senses that excel in their rocky homes, such as intensely keen hearing or the ability to feel life through vibrations in stone. Such creatures also would have evolved ways to navigate treacherous terrain, such as being expert climbers or being able to bypass steep rock faces via flight.
All manner of people can inhabit a planet’s subterranean environments. Visitors might encounter small, nomadic groups that survive by hunting cave-dwelling animals or by farming unusual crops, such as glowing mushrooms, verdant mosses, and stranger plants. The PCs may visit technologically marvelous metropolises that thrive underground by harvesting resources that can be found only in such environments: rich starmetal veins, minerals that thrum with magical energy, and even thinned boundaries between the Material and Elemental Planes. Such underground empires can thrive, becoming primary suppliers of materials that are difficult to obtain from aboveground sources. These civilizations might have developed technological advances fueled by both necessity and industry, and certain underground cities are likely to be rich troves of knowledge unlearnable anywhere else.
Regardless of a sapient civilization’s scale, it must contend with the dangers and threats that life underground presents. Nomadic groups could have developed gear to camp on sheer cliff faces, or cities might be carved into the stone to provide natural defenses.

Subterranean Adventurers

Source Galaxy Exploration Manual pg. 84
Although not every civilization can eke out a subterranean existence, those able to adapt to such locations can thrive, leveraging their underground surroundings for a variety of industries. Adventurers from such a place could be part of a mining company toiling beneath a planet’s crust, or they could be a scientist who studies cave-dwelling plants and animals. Subterranean regions also attract those who engage in morally gray fields. Poachers, smugglers, and others wishing to conduct shady dealings away from the prying eyes of the surface may find a subterranean environment the perfect location for a hideout or headquarters.
Adventurers who grew up in a subterranean environment likely developed skills that allowed them to thrive in their dark and stony surroundings. Perception is a key skill for those hailing from underground, and many adventurers must rely more on senses other than normal sight, such as darkvision and blindsense. Acrobatics and Athletics help adventurers navigate steep rock faces and unstable ground, while Survival allows them to find sustenance in a seemingly barren landscape. When facing threats, an adventurer can find great use in the Stealth skill to evade notice in the shadows. Adventurers who aren’t naturally adapted to cave environments might be from civilizations that rely on magic or technology to aid in their existence underground, and Engineering and Mysticism skills can serve such adventurers well.

Subterranean Worlds

Source Galaxy Exploration Manual pg. 85
Subterranean biomes can be found on just about any terrestrial planet, as they exist separately from the other biomes that occupy the surface, although their formation often relies on environmental factors in a planet’s history. Many cave systems form via erosion, whether from acidic rainwater dissolving stone, a glacier carving passages into bedrock, or even an ocean’s waves crashing against a cliff for centuries. Volcanic activity also results in the formation of subterranean environments, with lava tubes leaving intricate interconnected cave systems in their wake. In some cases, these systems may seem like separate worlds entirely from the rest of their planets.
While the sapient citizens of many planets often primarily inhabit the surface world, they may interact with the subterranean world for a variety of reasons. Perhaps mineral-rich veins crisscross the underground reaches of a world, leading a society to mine those resources. An abundance of plants and animals underground might lead a scientific community to explore the depths further.
On some worlds, the subterranean biome is essentially the only habitable environment for a sapient species, thanks to factors in the planet’s development. Perhaps sapient species left the surface world due to the prevalence of threats or a drastic change in the surface environment. A catastrophic threat, such as the emerging colossi of Daimalko (Near Space 74), would not only provide a reason for the sapient species of the world to retreat underground, but it also might inform how that society constructs its defenses and leverages its new subterranean home to defend against outside incursions.

Subterranean Rules And Reference

Source Galaxy Exploration Manual pg. 85
Several Starfinder rules are particularly important for subterranean adventures. Creatures that originate from subterranean biomes often rely on senses other than normal sight, such as blindsight or darkvision, and the rules for special abilities relevant to subterranean creatures can be found on pages 260–265 of the Core Rulebook. Adventurers from the surface world might not be adapted to an environment where natural light can be inconsistent at best. For these creatures, it’s important to understand the rules for light levels (Core Rulebook 261). Traversing the rocky terrain of a subterranean biome can be like navigating the hills and mountains of the surface world, and the rules for chasms and rock walls on page 397 of the Core Rulebook can be relevant as adventurers navigate the depths. Of course, the rules for falling on pages 400–401 of the Core Rulebook might come up frequently in a subterranean adventure.

Subterranean Toolbox

Source Galaxy Exploration Manual pg. 86
See Biome Subsections on page 46 for advice on how to use the following tables.

Subterranean Inhabitants

D%	Sapient	Threat
1–4	Amrantah	Acrochor
5–8	Bolida	Bloodbrother
9–12	Damai	Carnivorous crystal
13–16	Dragon, copper	Dust manta
17–20	Drow	Elemental, earth
21–24	Dwarf	Elemental, fire
25–28	Embri	Entropy slug
29–32	Formian	Ferrofluid ooze
33–36	Genie, efreeti	Herd animal, subterranean
37–40	Genie, shaitan	Magma ooze
41–44	Giant, slag	Murzzilat
45–48	Giant, stone	Odheo
49–52	Gosclaw	Predator, subterranean
53–56	Hobgoblin	Robot, guardian
57–60	Iztheptar	Robot, mining
61–64	Nuar	Robot, observer-class security
65–68	Planar scion, oread	Skreeling
69–72	Psacynoid	Skreesire
73–76	Quorlu	Surnoch
77–80	Shimreen	Swarm dredger
81–84	Telia	Swarm molitera
85–88	Thorgothrel	Thermatrod
89–92	Trinir	Thermophilic ooze
93–96	Trox	Thoqqua
97–100	Varculak	Writher swarm

Subterranean Adventure Hooks

D20	Adventure Hook
1	A group of local miners haven’t been heard from in days, prompting their loved ones to ask for assistance in finding them. Investigations into the mine reveal a strange sinkhole that reeks of sulfur and smoke.
2	A cult of the Devourer has claimed a section of a cave system as the site for a ritual designed to destroy the entire planet. Aboveground, a nearby town is terrified as tremors and sinkholes open in the surrounding area.
3	A geologist studying a crystalline cavern has noted unusual energy readings emitting from the minerals. Lately, the crystals have started shifting in and out of phase in a rhythm that suggests the cavern walls are breathing.
4	Multiple galactic corporations are vying for the mining rights to a rich starmetal vein recently discovered deep beneath a planet’s crust. Adventurers hired by one company to scout the area soon come into conflict with corporate interests from across the galaxy.
5	After a planetwide catastrophe, surface-dwelling inhabitants retreat underground with what supplies they can carry in a desperate search for a sustainable new home.
6	A copper dragon has recently laired near an underground town, becoming a source of ever-more-dangerous mischief. The town’s fearful inhabitants need help driving the dragon away.
7	A planar tear has led to an influx of fire elementals, threatening a nearby underground metropolis. The residents need help holding off the elemental incursion and closing the rift to the Elemental Plane of Fire.
8	Explorations into the deepest reaches of an extensive cavern system have led to the discovery of a gigantic metallic door, the composition of which scientists cannot determine. All efforts to open the door thus far have failed.
9	Unusually constant seismic activity has recently started causing extensive damage to an underground city. Xenobiologists suspect it may be due to the mysterious agitation of tunneling wyrms in the vicinity.
10	Local archaeologists have discovered ancient ruins hidden beneath the earth, and they’re eager to find adventurers willing to accompany them on an expedition. Unbeknownst to them, the ruins are anything but abandoned.
11	A local crime syndicate recently established a hideout in a nearby cave system, shaking down locals and stashing their stolen goods underground. With corrupt government and law enforcement providing no assistance, it’s up to brave outsiders to provide justice for the people.
12	Explorers inadvertently brought surface bacteria into a subterranean fungus forest, infecting the mushrooms that sustain an underground society. The town is looking for assistance in finding a way to save their crops.
13	The customs of a surface town forbid travel into a tunnel system nearby—but people in town have started to go missing, and several of the lost were reportedly last seen sleepwalking into the tunnels.
14	Pollution from surface power plants has seeped into the earth, causing toxic oozes to manifest near an underground settlement. Not only must the ooze threat be stopped, but the surface societies must be convinced to curtail their pollution to prevent the threat from returning.
15	Spelunking scouts have noticed their comm units picking up faint signals, with voices that sound almost like chanting, occasionally cutting through the static. The signals seemed to grow stronger the deeper they explored, prompting them to call for outside protection and assistance.
16	Smugglers have been using an underground tunnel network to ferry illicit goods between aboveground cities. Local governments are seeking freelancers to help track them down and put a stop to the illegal trade.
17	An underground city is hosting its first-ever extreme bouldering competition, where top athletes gather to tackle outrageous feats of climbing. The grand prize includes expensive gear and a hefty sum of cash, drawing attention from all over the system.
18	An enterprising shaitan freshly arrived from the Plane of Earth is looking to set up a gem-mining business in some nearby abandoned caverns. They seek employees and protection, though they dismiss the rumors of dangerous predators roaming the cave system.
19	A pod of dust mantas has been hunting closer and closer to a surface settlement, despite there being ample food for the creatures in their original, distant territory. The local government is hiring adventurers to investigate the reason behind the strange incursion.
20	Excavators recently discovered an ancient temple to Yaresa buried beneath old stone. The temple, seemingly abandoned long ago, contains strange machines fueled by long-forgotten magic. The temple may hold secrets to divinely inspired technology the likes of which the galaxy has never seen.

Archives of Nethys