Abandoned places are ripe for death by misadventure. Monsters, treasure, and items can all lure people to investigate and figure out what’s there or what went wrong. To that end, dropping clues is vital to intriguing an audience. Some of these places will be legendary while others are previously unknown. Both have their merits.
These can range from simple caves or tunnels, like a monster lair or dwarven home, to entire cities or even planets; if we want to be extreme, we can include solar systems or entire galaxies. The scale of abandonment suggests the scale of calamity that caused that abandonment, so choose accordingly. Danger is often assumed to lurk in such places, whether that danger is the new inhabitant, nature having taken over, or the remnants of the reason abandonment took place. Valuables are often assumed to have been left behind, attracting thieves and opportunists who might interfere with a band of characters going there.
Ruins are fun for audiences to watch people discover and explore. We can create mystery about what’s happening there now and what led to its demise. Clues and rumors should evoke curiosity and maybe feelings of dread, foreboding, wonder, and excitement. The more suggestive these are, the better, but it pays to have a reveal that goes beyond audience expectations regarding how cool the truth is, surpassing it. The trick to this is being coy about the truth, and inventing plausible variants on that truth, each one compelling but not as cool as the actuality.
But not everywhere needs a wonderful story. Disease, drought, climate change, natural disasters, and destruction in war are simple explanations that are the most likely culprits. Some places vanish for commercial reasons, such as over-exploitation of the resource (as in mining) or because a better commercial location usurped it, leaving a ghost town, which may have some residents after all.
Bear in mind how overgrown the location is. A rainforest or swamp quickly consumes a place so that it’s nearly impossible to find and less likely to be known; roads to it will disappear, too. More exposed locations will endure wind erosion and may become buried by sediment. An underground place won’t suffer much erosion but will instead lose structural cohesion from earthquakes, the toll of which we can tailor to our intended desire. Similarly, a mountain settlement might suffer rock falls. Sometimes people wonder why more dust doesn’t accumulate in abandoned places, but dust is partly particles from people, so no people means no more dust accumulation or the tracking in of other contaminants.
The durability of buildings is greatly impacted by their material. In less advanced settings, the roof is often the first part of a structure to disintegrate, allowing more rain and animals inside and speeding interior erosion. Stone structures could last centuries despite deterioration, but remember that locals might steal these materials for their own uses. With the advanced technology in SF, a location could last far longer, unaffected by erosion except for becoming overgrown and buried. Then again, maybe automated machines are keeping it pristine long after life has departed.
Magical or technological items are among the best ones left behind, whether by accident or on purpose. They are begging to be found by heroes, villains, or some random fool. Depending on the item, this could empower someone who shouldn’t have such power, raising them to king or overlord among their kind, such as goblins. Think of a weapon, armor, information found in books or scrolls, or a space ship or other tech, depending on your story. The object found might also be cursed. It doesn’t have to be the item responsible for the ruin existing. We might want to create a group of people who see it as their job to find, neutralize, and store these items to prevent such events.