So far throughout the series, two wells have been revealed to exist on the Island, although the Man in Black hinted that there may be others. These wells appear to have been created by an ancient civilization that existed on the Island and identify the locations of the Island's energy pockets.
History
According to the Man in Black, the wells were dug by hand by an ancient civilization that existed on the Island. These early inhabitants were confused as to why their compasses were acting so strangely in proximity of the energy pockets and dug the wells in an attempt to determine the cause. The Man in Black comments further that they learned nothing from their attempts - although this statement is rather at odds with the existence of the frozen wheel chamber. ("Everybody Loves Hugo")
Known wells
Orchid well
The Orchid well ("This Place Is Death")
The Orchid well was a structure built in ancient times that used to lead to the frozen wheel chamber. At some unknown point in the Island's history, a time-shifting Locke used the well to reach the wheel chamber. During this time, the well was located in a small clearing in the jungle - and had a rope descending into the depths. When seen again in 1974, the time-shifting survivors discovered it had collapsed. This well was located close to the location of The Orchid and within sight of the Statue of Taweret. ("This Place Is Death") ("LaFleur")
The Man in Black's well
The well that the Man in Black takes Desmond to ("Everybody Loves Hugo")
In "Everybody Loves Hugo", the Man in Black led Desmond to a second crumbling well which appeared to be located in a large clearing. Unlike the one near The Orchid this one was still intact as of 2007 and did not possess a rope leading down into the depths.
In demonstrating the depth of the well to Desmond, it took roughly 2.5 seconds for the Man in Black's torch fall to the bottom. Given that acceleration from gravity is 9.8 metres per second squared from a free fall at sea level and assuming the producers are attempting to be highly accurate, the well appears to be about 30 meters or 100 feet deep. The bottom of this well appears to be at least partially full of water.