Colorado Revised Statutes Title 39. Taxation § 39-6-112. Valuation of tunnels

(1) A tunnel excavated for the mere purpose of exploration and discovery of mines on the public domain shall be deemed to be real property and shall be listed and valued for assessment by the name thereof unless it becomes a part of a producing mine, in which case it shall be considered a parcel thereof. A tunnel excavated for the drainage of, or the exploration of, or for giving access to the mines of those excavating such tunnel shall be deemed an appurtenance to such mines. A tunnel excavated for the drainage of, or the operation of, or for giving access to mines of persons other than those excavating such tunnel shall be deemed real property and shall be listed and valued for assessment by the name thereof.

(2) Whenever any such tunnel not appurtenant to or a parcel of a mine is situated partly in one county and partly in another county or counties or in lesser political subdivisions, the valuation thereof shall be apportioned between such counties by allocating the total value thereof between such counties or lesser political subdivisions in the proportion that the length of the center line thereof within each such county or lesser political subdivision bears to the total length thereof.