Humanity's understanding of the space tunnels is very limited, but several peculiar traits have been discovered. Firstly, if Ship A enters Tunnel 1, exits Tunnel 2 and then turns around and enters Tunnel 2 again, it will emerge from Tunnel 1 again. Unless Ship "B" emerges from Tunnel 2 in the interim, at which point Ship A will instead emerge from wherever Ship B entered. (The single tunnel leading to World is #438.) Secondly, objects can only enter the Tunnel if they are below a certain mass, about 100,000 tons; anything larger will actually fit into the aperture, but will collapse and explode. The threshold of what the tunnel can handle is determined by the object's Schwarzschild radius. Finally, nobody knows how the tunnels work. Macro-level quantum entanglement has been proposed, but it is so far out of the realm of current physics that nobody believes it.