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Second Tiberium War
Third Tiberium War
By the time of the Third Tiberium War, however, subterranean vehicles had become untenable. Tiberium drastically changed prior to this war and massive growths of underground Tiberium began forming which were too dangerous to bore through.
Large swathes of the earth became too dangerous to cross, even above ground, while others required subterranean vehicles to make circuitous detours around any tiberium fields. These onerous restrictions led to subterranean vehicles being all but abandoned during this conflict. The Subterranean APC was replaced by the much speedier Reckoner and the Devil's Tongue was replaced by a more advanced version of the original flame tank.
Subterranean structures, bunkers and tunnels, however, did not need to travel around and so did not have this problem. With routine maintenance to keep tiberium growths away, tiberium generally posed no threat to structures unless it was part of a deliberate attack. They continued to be widely used throughout this war, examples include: the GDI's Ion cannon control and Nod's War factory and Redeemer engineering facility as well as the reinforcement bays.
By the time of the Ascension Conflict, subterranean technology experienced a renaissance and a massive revival. The Tiberium Control Network had cleared away uncontrolled Tiberium deposits, making the underground world safe for travel once again. Not only that, the Tiberium had conveniently left a vast network of caverns behind, ready to be exploited.
In short, Nod is free to rule the underworld a time once more.
Nod now relies on their vast subterranean network more than ever. It is used to deliver their Crawlers to battlefields, many of their units can now travel underground to avoid detection and conflict until just the right moment, and it forms the basis of several of their support powers such as the Risen Monolith and Subterranean Strike.
Amongst their burrowing units are the Tyrant, Scorpion tank, Spider Tank and Black Widow. The Reckoner can also burrow filling the roles of both the Subterranean APC and its predecessor. Several Defense Class structures can also burrow allowing for ambushes..