We construct the skeleton key to get into the aliens' hidden base.
Get the game here: http://store.steampowered.com/app/200510/
From the steam description:
XCOM: Enemy Unknown will place you in control of a secret paramilitary organization called XCOM. As the XCOM commander, you will defend against a terrifying global alien invasion by managing resources, advancing technologies, and overseeing combat strategies and individual unit tactics.
The original XCOM is widely regarded as one of the best games ever made and has now been re-imagined by the strategy experts at Firaxis Games. XCOM: Enemy Unknown will expand on that legacy with an entirely new invasion story, enemies and technologies to fight aliens and defend Earth.
You will control the fate of the human race through researching alien technologies, creating and managing a fully operational base, planning combat missions and controlling soldier movement in battle.
Strategy Evolved: XCOM: Enemy Unknown couples tactical turn-based gameplay with incredible action sequences and on-the-ground combat.
Strategic Base: Recruit, customize and grow unique soldiers and manage your personnel. Detect and intercept the alien threat as you build and expand your XCOM headquarters.
Tactical Combat: Direct soldier squads in turn-based ground battles and deploy air units such as the Interceptor and Skyranger.
Worldwide Threat: Combat spans the globe as the XCOM team engages in over 70 unique missions, interacting and negotiating with governments around the world.