Inside a secret Cold War bunker for Congressional leaders

President Obama says he's "sure" to cross paths with Russian President Vladimir Putin this week during the 70th anniversary of D-Day.

This comes as CNN has learned an April fly-by between a U.S. aircraft and a Russian fighter jet was one of the most dangerous passes in decades.

U.S. official: 'Dangerous' Russian jet fly-by was 'straight out of a movie'

Add in their differences over the Ukraine crisis - and the strain between the U.S. and Russia hasn't been this intense since the Cold War ended.

That's the subject of tomorrow's episode of CNN's original series, "The 60's."

Tonight, our David Mattingly takes you inside a secret Cold War bunker - meant to protect Congress in the event of a nuclear attack. It was kept secret from the public for decades.