Current information indicates that a massive amount of tunnels exist throughout the globe, sufficient to allow a vehicle as large as the Montauk to travel between combat zones while yet remaining underground. The two facts that Nod does use this system for even command personnel, (which indicates that they consider the system secure), and the fact that these tunnels extended enough for the Montauk to use them indicate that Nod has a monopoly or near monopoly on advanced tunnel boring equipment. 

By the time of the Third Tiberium War, the proliferation of underground Tiberium deposits have rendered these tunnels unusable. However, not only did the Tiberium Control Network made it safe for the Global Tunnel System to be used again, but it also added more tunnels due to the TCN removing Tiberium from underground and the surface, leaving hollow cavities in their place that was then used by Nod during the Ascension Conflict.

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Tiberium Universe Locations
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