Hidden civilizations are civilizations which the greater world either never knew existed or thinks died out long ago. They may be the last remnants of an ancient civilization. Often these civilizations are in remote or inaccessible locations, such as on mountaintops, deep within jungles, in deserts, or underground. There may be legends surrounding their lands, such as it being cursed, which may help to keep the outside world at bay. Often in fictional worlds hidden civilizations are encountered when outsiders search for new trade routes, seek hidden treasure, or attempt to take shortcuts through new lands.
Hidden civilizations may be aware of the greater world and may have hidden themselves on purpose. They may act as protectors of a fantastic land, item, or ancient secret.
When designing hidden civilizations, consider what drove them into hiding and/or and what elements have kept them hidden.
First contact between outsiders and a hidden civilization may be fraught with dangers.