One of the pools of water in the swamp south of Lumbridge has drained away, revealing the entrance to a new dungeon.
It is an extremely dangerous area, and those venturing in unprepared will not get far. You will not be able to see without a light source, but the candle seller outside the entrance will teach you how to make lanterns. Higher-level lanterns will allow you to avoid some of the hazards of the cave, but these have higher crafting, smithing and firemaking requirements.
Those who can banish the darkness will find the caves packed with new Slayer monsters to train on, plus the fearsome giant frog.