The Furthest Ring is the fabric of reality, containing all universes and planes in existence. In it, time and space are folding over each other, making it nearly impossible to navigate. Even if you do make it to another universe alive, it would likely be one you weren't looking for at all and in a time period far away from where you started. Luckily, if you get lost, chances are Bugs Bunny will find you and take you home.
Entering the Furthest Ring is extremely difficult. One could do it by finding a rip in the fabric of time or space and hoping for it to lead outside their own universe. Another way would be finding the Edge of the Universe, and moving at higher than light speed toward it.
Since the Furthest Ring can lead to any universe, being able to navigate it is an ability extremely sought after. If you have great knowledge of either time or space, you can predict the path you need to take to get to your destination. However, it may take years to fully travel the journey. Luckily, if you have a time machine, that doesn't matter.
Another way to navigate the Furthest Ring is with the Furthest String, a string which can latch on to any Universe and guide the user through the twisted space of the Furthest Ring.