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. An Alien Base Assault will remove the threat but should never be taken lightly.
The construction of alien bases are marked by heavy UFO activity in rural areas. In addition to the scouts and Supply Ships sent by the aliens to survey the potential base site, a Battleship is sent to inaugurate the base as soon as the construction is completed. Just like Alien Infiltration, shooting down the UFOs will delay but not entirely stop the process. To be precise, the moment the Battleship appears in orbit means that the base is fully operational. So, if a Very Large UFO appears as part of a large force of Small and Large UFOs, then it is likely to be on an Alien Base mission, and is likely to make a landing. It is possible to capture the Battleship's commander and 4 power plants intact at this time. It is much easier to find the commander on the bridge of a Battleship than deep inside the maze of an Alien Base.
Note that shooting down the Battleship will not prevent base construction, so if you see a Battleship on an Alien Base mission this is a signal to start looking for the base.
Once successful the aliens will score 50 points, and an extra 5 points per day which is eventually subtracted from your score. Alien Bases add significantly to the Alien score when they are constructed and between that and the much higher UFO activity present will have neighbouring countries in particular very agitated, possibly leading to them reducing or withdrawing funding. As such Alien Bases in key areas will need to be dealt with somehow, while Bases in remote locations might well not be so critical to target for action immediately.
