Cold War

The Cold War is a period in Earth history (specifically after World War II) when humans dug a lot of bunkers. This was apparently due to post-war disagreements between the United States of America and the Soviet Union.

Transformers: Prime
Autobot Outpost Omega One was originally a Cold War nuclear missile silo. After the 'Bots came to Earth, the silo was reconfigured for their habitation.

Transformers: Rescue Bots
At some point during the Cold War, a bunker was built under the fire station on Griffin Rock. After the Rescue Bots were assigned to the island as protectors, they made use of it.