During an unknown period of the Island's history, but prior to the construction of the Orchid, a stone well with a rope leading into it existed near to what would one day become the Orchid entrance. After Locke bid goodbye to the time-jumping survivors, Sawyer helped him down the well. Below the well was a cavern that led to the frozen wheel. This was a separate route to the wheel than the one Ben used. After another time flash (which seemed to emanate from the bottom of the well), this well vanished, sealing Locke inside the wheel chamber. Charlotte, alluding to a time in her childhood when she was associated with the DHARMA Initiative, knew of the existence of the well.
Just warning you, that if you haven't watched the whole show might see some information before you are ready. To help with that we have this nifty filter. Just set it to the episode you you are upto, and we'll do our best not to spoil anything.