Alien Missions
Alien Research
The Alien research mission is used for collecting basic data on Earth and its inhabitants. Small vehicles are predominantly used, with occasional landings in deserted areas. This type of Alien activity poses the least threat to X-COM, with little concern from governments or the public.
Alien Harvest
The Aliens have many uses for Eearth's fauna. Animals are abducted secretly, and returned with various organs removed. Cattle mutilations are predominantly reported along with UFO sightings. This type of Alien activity causes great concern by governments and considerable anxiety amongst the population. This type of activity occurs mainly in farming land. The theory behind the 'Alien Harvest' suggests that Alien races originally "seeded" the planet with its flora and fauna, and now they have returned to reap the harvest they have sown.
Alien Abduction
This is the most insidious form of Alien activity. The abduction by Aliens is widely reported, despite the Aliens' attempts to erase the experience from their victims' memories. Abductees report being subject to humiliating physical examinations, including impregnation of alien foetuses and bizarre genetic experiments. The purpose behind this activity appears to be linked to genetic mutation and manipulation of the Aliens own genetic material. This activity causes great alarm, and occurs in populated areas or cities.
Alien Infiltration
Earth governments can be infiltrated by Alien agents which are Human in appearance. This can result in official contact between Aliens and governments at the highest level. The climax of this activity is characterised by intense UFO activity in the vicinity of major cities. The aliens will attempt to sign a pact with an earth government by offering knowledge of their superior technology. In return, the government will allow the Aliens to conduct their activity unhindered. This Alien mission represents the worst threat to X-COM. If a government agrees to a pact then its funding will cease.
Additional Information
Four UFOs take part in an Infiltration mission, landing in both the target and neighboring countries. Limited information indicates two of these are often Battleships which must be shot down to forestall the infiltration. Engaging intact UFOs on the ground is apparently not enough to prevent them from completing their mission.
Zombie says, "According to my preliminary results, Russia cannot be infiltrated in the CE version of X-COM so losing the game by this method is impossible." (Jun 16, 2005, StrategyCore forums)
Alien Base
Aliens will construct secret underground bases in remote locations. After some initial reconnaissance flights some intense UFO activity will occur as the base is being built. These bases are known to contain experimental labs for human abductees, and supplies for further activity in the region. The presence of Alien bases will generate a large amount of reported Alien activity without the presence of UFOs. In order to locate a base, an X-COM craft must patrol an area for a few hours to stand some chance of detection.
Additional Information
is there a limit to how many bases can be in a game at once (30 so far)
Alien Terror
When the Aliens terrorise a city, they will deploy some special forces with awesome powers. Civilians will be directly threatened, and governments will be forced to evacuate whole areas. The main purpose behind this activity is to generate sufficient public hysteria so that governments will threaten the X-COM project.
Alien Retaliation
If X-COM Interceptors are being particularly successful in shooting down UFOs then the Aliens may take some retaliatory action. This could result in a direct attack against an X-COM base. However, the aliens have to find an X-COM base in order to attack it, and provided UFOs are kept away then there should be little danger of an assault.
Alien Supply
Once an Alien base is constructed, then it is resupplied on a regular basis by a special supply vessel. If one of these vessels is detected while landing then it is certain that an Alien base is nearby.