The Hub was a dimension on a seperate server, testalpha.minecraft.net
1, featured in several versions of the 16.05 branch.
The Hub looks like a maze of 3x3 corridors. Its walls are made out of Dimension Wall blocks with occasional Quad Windows and are decorated by torches. In the later appearances of the Hub, there were occasional Greenscreen blocks in the walls and in the floor next to water puddles.
The corridors may end in a dead end or in a room. The dead ends are usually made out of Grenscreen blocks, but some dead ends have iron doors. The room always has a pool of water next to an iron door.
It also seems like the Hub had some "exterior" space. Those pieces of land were made entirely of Fake blocks, as seen in Internal footage 3.
Alternative hubs are completely different from the hub's appearance mentioned above. While it remains the same 3x3 corridors with random dead ends, it also has big rooms, weird looking trees and parallax effects.
These rooms are said to be used in *special* occasions, su ch as player unsuspension or recruitment.
The Hub allowed players to travel between each others' worlds without connecting to them directly. It worked like modern BungeeCord, but instead of additional worlds on the server, there were actual players' worlds. One could connect to a world of any other player, if both players were connected to the Internet.
Iron doors served as gateways, which allowed to enter or exit the Hub.2
The Hub was first seen in Version 10, but Internal footage 2 implies that it existed at least as early as Alpha 1.0.16.05_07.1
The next appearance of the Hub was in Internal footage 2. dev6 fixes a bug related to it.
The final appearance of the Hub was in Internal footage 3. dev6 and dev4 used it to travel to some player's world.
The existence of the Hub was hinted at in the video Attempt to run today.
Footnotes
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The meaning of iron doors in videos that didn't explicitly feature the Hub is yet to be discovered. Those videos are Version 03, Version 05, Version 06 (may mean that the Hub did exist on Alpha 1.0.16.05_06) and ????. ↩