Subzero is a central transport hub placed in the exact centre of the map, at co-ordinates x = 0, z = 0, below Zero Zero. It is a glass structure underwater, connected to the Spawn Island via the Underwater Railway, the Big Silly Bridge and the Nether Hub. There's a button next to the nether portal for teleporting back up to Zero Zero on the surface.
It contains the Teleport Hub, and is expanded eastwards as people purchase teleport blocks for inclusion in the Teleport Hub.
If you're going on foot and by boat in the overworld, you can either head for Spawn Island and then go for the Big Silly Bridge, or you can use the debug screen to navigate to the very centre of the map. Subzero is accessed by falling down through the hole in the centre of Zero Zero.
If you're at spawn, the Underwater Railway is the quickest way.