The first time I played Terraria hard mode, the corruption overran my Jungle. Corruption does that. Corrupted purple grass grows along the surface of dirt, and ebonstone transforms nearby stone into more ebonstone. Furthermore, purple grass and ebonstone dry out mud, turning it into dirt.
Jungle and Shroom biomes depend on mud, which means they die out rapidly when exposed to the corruption. And all efforts to purifying the jungle back only turn it into common forest. It was possibly the best environmental message I’ve encountered (certainly the best I’ve seen in games). My surface Jungle was entirely lost. Just corruption as far as the eye could see.
Fortunately, our Underground Jungle still had parts intact, but the corruption was still spreading and fast. My Terraria partner and I followed along the spread perimeter trying to create a contamination break. In order to combat the high spawn rate of monsters (some of which were hard-mode tough) we created a network of rooms with doors that we closed and barred behind us.
Eventually, our perimeter grew really long. It wound deep into the ground and meandered through the jungle territory.
And that’s when I realized this was a damn good justification for a dungeon.
Not a dungeon as in the castle lockup, but an underground complex,* preferably one that goes on and on. This is the mainstay of Dungeons and Dragons. It’s right there in the title. Only in the 1990s did above-ground adventures become more common than the ever-weaving underground labyrinth of tunnels and caves teeming with monsters and loot. In the old days, the surface was safe. Adventure and danger were to be found in caves and sewers and catacombs. Monsters were confined to the dark and dank places.
The reason for this love of underground labyrinths is that referees could only plan and pre-map so much, and prepared story notoriously fell short of filling out an entire weekend’s gaming binge. And then, underground cavern- and hall-networks could be easily improvised, so long as you chose one cardinal direction (say, West, or down) and confined your expansion to not going in the other one (never bend back East / up) The reasonless, meandering tunnels stayed interesting, and fun enough to keep the game going.
Eventually, gamers started raising the obvious questions, like who has time to carve all this out anyway?.**
That’s the problem. Other than the occasional tomb-of-dead-kings or lost-and-buried city, there’s not so many reasons to hollow out large mountains for adventurers to loot.
Tolkien’s answer was the Dwarves, greedy for Mithril, hence the endless and labyrinthine complex that is Moria. Nothing like mines dug by obsessive Dwarves to provide for unlimited exploring opportunities.
Our modern world offers underground municipal complexes which I suspect start by having commercial or municipal buildings that are connected directly to underground rail systems. The Pentagon in Washington DC not only has considerable underground facilities of its own but has its own underground Washington Metro station. The Metro 2033 franchise capitalizes on underground metro-rail tunnels serving as an outstanding place for adventure (and to survive nuclear war).
And now, we have the Terraria corruption containment barricade.
Were I writing fiction (there I go again), I’d start our adventurers in old-growth wildlife territory. Some say it’s virgin, yet there are ruins of ancient custodial buildings erected by some long lost civilization. Still, no-one’s been around here for over a century. Few sages know of ancient imperial texts that speak of these wilds as a sacred garden that underwent restoration.
Such texts speak also of this land being forbidden, but our adventurers would heed no such warning. Vague dangers better left undisturbed are no match for this band of irregulars. A wanderer sighted a vast fortress doubtlessly teeming with untold riches and powerful weapons. And given the coming war, untold riches and powerful weapons are exactly what we need.
And then the fellowship finds the facility.
It’s hard to miss. Foreboding spiky architecture. Towering ramparts and fortifications adorn every hillside, continuing on endlessly in either direction until eventually vanishing behind the rises. This isn’t just a fortress, it’s a lost city!
Our glory-seekers are already dreaming of ancient riches when one of them finds an etched placard. The letters are of the ancient alphabet. But there are also odd, unrecognized geometric icons. Translated, it reads This is not a place of honor. No esteemed deed is commemorated here. Nothing valuable is stored here. This place is still dangerous. Leave this place alone.
Of course, they have to go in.
* I assume dungeon refers to an underground vault, and was commonly used for locking away prisoners. In French, a keep is a donjon. But this is dubiously related to the English dungeon. The French term for underground prisons is the grimly romantic oubliette meaning forgotten place, or place of forgetfulness. It’s the place that someone puts you when they want to forget about you. (Related: obliterate, to wipe something out so that it is forgotten.)
** One of the lesser-asked questions though pertinent, is what’s holding all this up? The further down mines go, the more of the earth is load-bearing and can’t removed without either serious pillaring or serious disaster. If your dungeons run particularly deep, stress pressures will quickly outweigh conventional technology, and Dwarf magic will have to be implemented. If you ever wondered why super-deep dungeon construction is powered by the souls of forsaken children (and the pillaring gets shinier and fancier the further down you go) this is why.
Some wizards / Istari / Valar / lesser deities-who-meddle-in-the-affairs-of-mortals will simply reserve a part of their own life energy to sustaining their super-high / super-deep construction works. Hence one of the dangers of adventuring too far is found when our delvers eventually encounter and fight the being holding it all up. After a short-lived victory, they find themselves underneath a collapsing mountain, where no living thing has a right to be. It’s the easiest excuse for the referee to end the game once and for all.