They say keeping a secret is a lost art, but that clearly doesn't apply to everyone. Some classified information has remained that way since time immemorial, known only to a select number of people: two, in fact.
These are some of the world's best-kept secrets, but to what extent do they relate? The codes to America's nuclear arsenal? Apple's plans to break into the self-driving car sector? Nope. In many cases, it's far more trivial than this.
You might be surprised to hear that recipes within the fast food and soft drinks industries are probably known to fewer people than either of the above, and the same applies to cryptic information about popular songs and albums released decades ago.
In some cases, these secrets are released to the public when the world is ready to hear them, but more often, their two custodians pass the information on to another chosen duo before they pop their clogs, and the cycle continues.
The truth is that many of the world's best-kept secrets are completely bizarre, and you're unlikely to ever find out the truth behind them in your lifetime, unless you happen to be one of two specific individuals.