Many members have contributed to this topic of "Underground Bases". Plenty of details, theories, and facts as to the reason(s) for most countries, governments, as well as the wealthy to secure underground living quarters.
So here is another strong possibility....
. . . The Fatima Prophecy, however, refers to only 3 days of darkness. In discussions he has had with other people about the apocalypse, Padre Avontios does mention three days of darkness which occurs in the beginning and which he considers to be a particularly dangerous period during which time people should not go outside of their shelters. . .
. . . When you see these things come to pass, you must close yourself into your own house and keep the windows and doors closed and never open them or go out, or you will die. For if such occurs, you would expose yourself too greatly to these conditions, even to the radioactivity itself, from the cosmic-solar event caused by the Earth and the sun encountering the area of concentrated cosmic particles and materials in space, and the response of the Sun thereto. . .
http://etheric.com/future-reading-padre-avondios/