Discovered, a few months ago, buried in a box of other documents deep in the archives - the Declaration of Independence. It's one of only 26 surviving original copies of the document that were printed in Philadelphia on July 4th, 1776.
"It was found recently interspersed within our colonial records actually relating the American War of Independence," said Edward Hampshire, a curator at the British Archives.
The war had already been raging for a year when the Declaration was signed and about 200 printed copies - the exact number isn't known - were sent around the fledgling United States to rally the troops and citizenry to the cause. Sometimes it was read out to crowds by riders on horseback.
There's still debate about where the ideas came from, but the language was unmistakably American and still defines the democratic ideal:
"Of truths that are self-evident that all men are created equal."
"Of inalienable rights to life, liberty, and the pursuit of happiness."
The last time one of the 26 surviving copies emerged was 9 years ago when it was bought for more than $8,000,000 by a consortium headed by TV producer Norman Lear. All but three of the copies are in the United States. The others...?
Right here in the hands of the old enemy: England.
"It is ironic now that actually we have 3 of these now - which is more than the U.S. Library of Congress," said Hampshire.
But they say it's in good hands.