Nuncio

Nuncio's statue. Art from SS.

Nuncio: where mailmen go when they get frustrated, bored, or ashamed of what they do. All lost or refused mail ends up here, and the mailmen try to deliver it. The statue in the middle changes, according to what culture is living there. The currency: dead rats.

At the back of the post office, a pit reaches several hundred feet down, seemingly without end. The post office workers avoid it. There is a large needle marked with Correspondence sigils at the bottom of the pit, which is so far down one can't even see the entrance.

Nuncio may have been made by the Bazaar, or something similar to it, to guarantee no message is undelivered or misunderstood. 

Fun fact: The basic idea for Nuncio was created by Spacemarine9, creator of the lore blogs Saint-Arthur and Saint-Beau.

Origins

While Nuncio isn't a place, it is a title...

Nuncio (officially known as an Apostolic nuncio and also known as a papal nuncio) is the title for an ecclesiastical diplomat, being an envoy or permanent diplomatic representative of the Holy See to a state or international organization. A nuncio is appointed by and represents the Holy See, and is the head of the diplomatic mission, called an Apostolic Nunciature, which is the equivalent of an embassy.

This perhaps is referencing the giant statue in the center of the island. The ultimate messenger of the Unterzee, Nuncio is sanctuary for all those who must deliver packages and messengers. It is only natural the island is named after one of the most important messengers in the Christian world.