While isolated cases were seen during the First Tiberium War, it wasn't until the Second Tiberium War that they were cataloged and properly analyzed. The veinhole is a subtype of Tiberium, but unlike the mineral itself, they do not leech the soil of nutrients, but rather grow over it completely. They always originate from a central mass, with a "mouth" of sorts accompanying other animal-like qualities. The mouth is where everything that is destroyed by the veinhole ends up, to be consumed.
The veinhole monster is extremely sensitive. It is highly agitated by vibrations and kinetic energy and will tear apart anything that disturbs it with pseudopods formed from vein materials. Hover vehicles and infantry can usually cross safely before the creature becomes angry.
For self defence (should the central mass be threatened), the veinhole expels clouds of highly concentrated Tiberium gas that float around the mouth and destroy attackers foolish enough to come into contact with them. It does it with good reason, as the central mass is the heart of the creature - should it be destroyed, the whole mass slowly withers and dies.
Veinhole monsters were located in areas which were later classified as borderline Red Zones, due to their dangerous mutation levels and Tiberium proliferation. For example, at least one was present in the Genesis pit area, and two were present in the South American Region that Ghostalker investigated.
The Brotherhood of Nod used veinholes by collecting veins through the use of a 'Weed Eater' vehicle. Once enough veins were collected, the processed gas, similar to veinhole gas, could be turned into a Chemical missile, which released clouds of Tiberium gas that were very similar to the ones released by a veinhole in self-defence.
Tiberium veins will damage and destroy most vehicles that drive over them, but will not harm infantry (excepting jump-jet infantry). Vehicles that are not affected by the veins include the Wolverine, Attack Buggy, Hover MLRS, Harvester, Weed eater, as well as aircraft. The veins expand constantly, and will also destroy structures which they come into contact with, though the expansion can be blocked with concrete walls. Killing the veinhole monster will stop the growth of the veins and cause them to recede over time, and this is most easily accomplished with a small group of Wolverines or Attack Buggies. Attacking the veinhole will cause it to release damaging gas clouds, although these can be easily avoided. When playing in campaign missions or skirmish mode, the veinhole can often be used to your advantage as computer-controlled units will often disregard the danger posed by the veins and drive right over them, allowing you to lure them to their destruction.