Stairs in woods
Originating on /r/nosleep, a common trope in stories about exploring deep in the woods is mysterious stairs leading to nowhere and with no noticeable signs of a previous structure. They're usually found in the context of searching for a missing person 10 or 30-40 miles into the forest by a professional rescue team. When approached, narrators relate either an overwhelming sense of dread or see glimpses of strange creatures nearby. Supposedly, park rescue teams consider this strange phenomena a common occurrence and have been instructed by their superiors to never approach, take photos, or talk about their existence.
The style and apparent age of the stairs vary wildly between stories, from masonry so old it's starting to turn to dirt to stairs that looked brand new, "copy-pasted" from a suburban home. In at least one case, the stairs were upside-down, with the steps on the underside. Another had a staircase seemingly from a lighthouse, metallic and spiralling tightly upward. Many stories also report there being a well-maintained grave or graveyard nearby, despite how unlikely it would be for someone to come so far into the woods to maintain it.
The veracity of these claims should be taken with a big grain of salt, as /r/nosleep is a forum intended for fictional creepypastas presented as truth with an implicit suspension of disbelief to better engage with the story. However, since their appearance on /r/nosleep stories have popped up in other places as well, seemingly dropping the facade of telling a ghost story to relate real strange experiences.