At the very bottom of the CC is the second-level basement, where the Nether portal is located. Nether brick and fence are used decoratively to add some thematic continuity to the other side.
The original Nether portal sat on a tiny island in what would become the CCA bay, from the map's inception in early August 2012 until early December of the same year. Once it was decided the bay would contain the Seaport, the decision to move the Nether portal was an obvious one.
While the portal itself is finished, where it exits into will change dramatically in the future.