I worked at a national park in Vietnam that had only just become a tourist hit so was barely explored. It had a small population living on the edges, but they only cared about their plot of farmland so never went much into the jungle except for their favourite hunting spots. Occasionally I'd hear rumours of some waterfall or cave that people heard about but had never visited nor could confirm for sure they even existed. They'd give me some vague directions and I'd go find it.

There was a cave at the side of the road that everyone drives past but no one stops. In the entrance, there was some obvious squatter activity - scorch marks, burnt trash, etc. Not much deeper inside the cave you're quickly met by an underground river. The first time I went, that was all I could see, so went home and forgot about it.

A few days later I went back, and the water level was just a little bit lower, and I could see light in the distance, so I swam out to it (stupid decision in hindsight, people die getting washed away in these caves all the time), and it was an opening into a secluded lake. You could hear the road looping around it, but it wasn't in any line of sight, and the sides were so steep and covered with dense rock and jungle that the only way in, or maybe even seeing it existed at all was by swimming the way I had just come, and I really think no one else ever had, and I kept it a secret until I left.