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The Silo (also known as the Er'cana City Silo or Uran Silo) is a small building above the lake, on the wall of the D'ni cavern. It is used as a testing place for the pellets constructed in Er'cana.
The building consists of a dome of sorts, with inside it two balconies overlooking the pool on the bottom of the Silo. The link-in spot is located on the lower balcony, along with a 'pellet meter', which apparently measures the quality of the pellets and displays this visually, then drops it into the pool, in which the effect of the pellets on the water of the lake can clearly be seen. This pool is supposedly connected to the rest of the lake. The only usable exit is a Linking Book back to Er'cana on the lower balcony, though there is a closed door on the upper balcony.