Eventually the Aliens will begin construction of a Alien base somewhere on Earth. This process takes time and is marked by a significantly increased UFO activity near the area the base is being constructed.
A base will be constructed once the last construction ship lands and finishes the work, once this ship takes off the base will be operational. Once the construction ship has landed the only way to stop the alien base from being finished is to assault the UFO before it takes off.
The only way to remove a Alien Base is to launch an assault on it .
Alien Construction missions are avaliable mid way through the third month.
In order of apperance