Monster Virtualization is the process of Monsters spawning. There are variety of ways that monsters spawn.
Standard Virtualization
Standard Monster Virtualization is by far the most common form of monster virtualization shown. This is usually run by X.A.N.A.. The monster's vector lines appear, and then the area between the vector lines is filled, and the monster either falls to the ground or flies.
Spawning From The Ground
This was only shown in "Mister Pück". This is a means by which Bloks can spawn. Basically, the ground cracks, and monsters crawl out of the cracks.
Manta Hatching
Mantas can spawn from the outer walls of Carthage. This seems to follow a time delay cycle. During Tidal Wave, a similar phenomenon occurred with a Creeper.
Manta Mine Laying
Mantas can lay Flying Mines. The largest occurrence of this was in "Cold War".
Monster Materialization
On two occasions, X.A.N.A. materialized monsters via the Scanners. This first happened in "False Start", where X.A.N.A. materialized a small army of Kankrelats. Later, in "A Bad Turn", 3 Krabs were materialized, though doing so caused the scanners to be destroyed.
Monster Spectres
On a later occasion in "Hard Luck", via adapting the teleportation procedure used by the Lyoko Warriors, X.A.N.A. created monster Spectres. These were in the form of Kankrelats.
Kolossus Virtualization
The Kolossus has been virtualized twice. Both times had the same, albeit unique, procedure. In this variation, hundreds of Replikas start allocating their respective resources to the creation of the Kolossus. It then emerges from that Sector's Digital Sea. The first usage of this revealed the sheer number of replikas to the Lyoko Warriors, which led Jeremie Belpois to develop the Anti-X.A.N.A. Program.