Subterranean

Subterranean refers to underground. In particular it refers to Nods Subterranean technology which allows them to dominate the underground much the same way that GDI dominates space.

History
Underground structures and tunnels had existed long before even the First Tiberium War. Bunkers and tunnels had long been widely used and throughout the Tiberium wars many buildings had all or part of their structure below ground to protect them against attack.

Second Tiberium War
Subterranean vehicles, however, were invented by Nod in between the first and Second Tiberium Wars as a counter to GDI's growing dominance of the skies. This was to allowed them to operate during the Second Tiberium War unmolested by GDI's Ion Cannons or their superior air force. Once underground only Mobile Sensor Arrays could detect them.

They took the form of the Subterranean APC, which was used to ferry troops undetected across all terrain, and the " Devil's Tongue flame tank" which could pop up amidst the enemy, crushing and immolating everything in range.

They also had an underground command vessel, the "Montauk" which travelled around though their Global tunnel system.

The GDI response was to deploy Mobile Sensor Arrays to detect incoming Subterranean units and pave over the ground in and around their bases with concrete which was impenetrable by Subterranean units (and also allowed units to move faster).

Third Tiberium War
By the time of the Third Tiberium War, however, subterranean vehicles had become untenable. The massive growth of underground Tiberium deposits made large swathes of the earth too dangerous to cross underground requiring circuitous detours around any tiberium fields. These onerous restrictions led to subterranean vehicles being all but abandoned during this conflict.

Subterranean structures and bunkers, however, did not need to travel around and so did not have this problem. They continued to be widely used. Examples include the GDI's Ion cannon control and Nod's War factory and Redeemer engineering facility.

Fourth Tiberium War
By they Fourth Tiberium War Subterranean technology experienced a renaissance and a massive revival. The Tiberium Control Network had cleared away uncontrolled tiberium deposits making the underground world safe once again. Not only that the tiberium had conveniently left a vast network of caverns behind ready to be exploited.

Nod now relies on their subterranean network more than ever. It is used to delivery their Crawlers to battlefields across the world and the basis of several of their support powers such as the Risen Monolith and Subterranean Strike.