Digging around in the sandpit a bit more, you discover an iron ring. You try unsucessfully to lift it, but it seems to be attached to something. Further digging reveals it to be connected to a large oak platform.
A lot more excavating reveals the object to be a wooden trapdoor, buried beneath the sandpit. You decide that nobody must have discovered it for years. What unknown thing awaits you underneath it?
Unable to contain your curiosity any more, you grab the ring and pull..