Keep out!

You have to read this book - 100 places you will never visit : the world's most secret locations by Daniel Smith  - as the chances of actually visiting the places listed is not possible/highly unlikely/illegal...

Ever wondered what it takes to get into Fort Knox? Fancied a peek inside the Coca-Cola Safety Deposit Box? Would you dare to visit Three Mile Island? The world is full of secret places that we either don't know about, or couldn't visit even if we wanted to. 

Gaza tunnel

Sometimes they lie under our feet: London's Whitehall tunnels, Moscow's hidden subway, and the Gaza Strip smuggling passages, or they are so confidential that their existence is not even acknowledged by governments: the USA's Mount Weather Emergency Operations Centre, and Mount Baekdu in North Korea a mountain lair purportedly built by Kim Jong-il.
 
A series of tunnels has been dug beneath the barrier separating Egypt and the Gaza Strip (a border that has long been subject to an Israeli blockade). The tunnels are used for smuggling people, transporting illicit food supplies, and contraband ranging from arms & explosives to recreational drugs.

Now you can glimpse the Tora Bora caves in Afghanistan, visit the Tuscon Titan Missile Site, tour the Vatican Archives, or see the Chapel of the Ark. 
 

The seal in the lobby of the headquarters building

The only secure way to see these shadowy and mysterious facilities, this fascinating guide book takes a look at 100 places around the world that are either so hard to reach, so closely guarded, or so secret that they are virtually impossible to visit any other way.

The US Central Intelligence Agency in Langley Viriginia is among the most secure buildings on the globe. The CIA is responsible for providing intelligence on matters of national security to Washington, and operating a network of spies around the world.

 
There are three Australian entries - the Woomera Rocket Range, Pine Gap, and the headquarters of the Joint Operational Command. Why can't they be interesting sites like the Amber Room, the Vatican Secret Archives, or the Skywalker Ranch instead of scientific-military facilities.
This is one time when reading the book is better than actually being there...
100 places you will never visit : the world's most secret locations by Daniel Smith.