The Underwater Tunnel is a fixed location to the south west of the starting zone added during the Update/03.07.2013. It can be upgraded ten levels, all granting a higher increase to the Population Cap and gives access to the fourth expansion territory.
As with all construction projects, there is a choice between waiting, and receiving gifts via neighbors, or buying any/all required assets with megabucks . A total of 375 assets are needed to complete construction of the Underwater Tunnel to level ten; this equates to 375 if you were to buy every asset.
Once the expansion is unlocked, the player is given the choice to build: