"This is the end."
"Two vast winged shapes guard a gate of something like resin, smooth but uneven. It is deep gant - the colour that remains when all other colours have been eaten. Ice crusts over the crack between its valves. Approach, and your breath freezes, falls tinkling in shards from the air. It would be utterly foolish to touch the thing.
A merciless wind blows from everywhere to everywhere. It passes without effort through your bridge-coat, your flesh, your lungs. The dock lies empty."
Avid Horizon is the northernmost known place in the Unterzee - as in, travel North far enough, from anywhere in the Zee, and you'll find yourself gawking at the guardian statues. It's bitter cold there. By 1900, nobody's gone further. Dozens have died trying. Zailors, of course, are advised not to go.
One of the docks there offers those who have been exiled from London a chance to beg for forgiveness, but they must be willing to sacrifice everything, and there is no guarantee the Admiralty will choose to accept the apology.
The realm of the Judgements lies beyond the gate. Passing through said gate strips a traveler of their body, and they must somehow forge a new one while inside.
By 1906, the British Empire has colonized the High Wilderness, abandoning London and establishing Albion. The Avid Horizon apparently served as the Empire's gateway into the heavens, as it was there that the Parzifal, the first spacefaring locomotive, was built and launched.
Sometime afterwards, the gates of the Avid Horzion have been sealed by Her Renewed Majesty. However, a sea has since formed in the High Wilderness under the gates. Of all of the possible explanations, one is that this small Silent Sea is a part of the Unterzee that poured into the sky when the doors were opened (would that make it a Zilent Zee? The Overzee?)