Mr Light wrote:
^^ The tunnel is filled in during the 70s (Sawyer's rope going into the ground). In order to reach the Donkey Wheel, they need the construction capabilities of the Dharma Initiative to dig down through the earth to get to it.
No, the rope into the ground was way back in the past at the time of the statue. In the 70's there is a well there, but the top of the well has been capped with stone blocks a few feet down. Remember Locke goes down the well (sometime in the 50's we presume) then there is a flash and the well is gone in the ancient past (the rope going into the ground) then there is the last flash taking the Losties to the 70's and the well is back, but capped. So sometime between the '54 and '73 someone caps the well. Possibly to prevent people from gaining access to the wheel. (Maybe after Widmore used it to move the Island (if that's what happened to get him exiled.)) We also know that the whole well wasn't filled in because in the present when Ben went down and moved the Island the tunnel would have been filled with dirt (also if the well was the only access to the wheel, and the wheel is the only wway to move the Island to safety I can't see the Hostiles permenently sealing it away. Capping the well is the best way to prevent people from getting to the wheel, while still giving them access later on if needed. If those stone blocks were removed they would probably still be able to repel down to the tunnels where the donkey wheel is.
But either way, Dharma didn't use the well or seem to even know about the wheel (until the scene in the first episode of this season where Chang sees the (thermal?) image of what is behind the rocks given to him by the construction guy). They just built the Orchid near by the donkey wheel because they must have noticed strange events going on it the area. But we have seen that they had no idea about the wheel otherwise the station would have included it, rather then Ben having to blow a hole in the wall to reach the tunnels beneath.
Now, as to how Ben and the Hostiles knew about it, well that still has to be shown.