Does that mean that the underground installations are not fully known?

According to a secret portion of the Pentagon study, more than 70 nations now use underground installations.

Tours of the above-ground and underground installations around the missile are conducted daily.

The site indicated was near the largest of the alien underground installations, in a barren area.

But I guess you didn't know anything about the underground installation, way back then.

Diaz finds himself in a massive underground installation created by the aliens under Manhattan.

He punches his way into Zod's underground military installation and confronts him.

From here it will suck up waste from an underground installation.

Observation posts, communications centers and underground installations were also added in the sector.

It always looked like you were on your way to a fallout shelter or a secret underground military installation.