Before John Quincy Adams became President, he was the greatest American diplomat of his time. He helped negotiate an end to the War of 1812, helped articulate the Monroe Doctrine and (as Secretary of State) negotiated the acquisition of Florida. Later in life, while serving in Congress, he was a very early opponent of slavery.
So, what could possibly be a stain on this illustrious life? Well, Adams had a confirmed belief in something called the Hollow Earth Theory. If you think today's conspiracy theories are loopy, get a load of this one. The theory held that Earth is not a solid rock—but instead is made up of concentric layers, separated by empty spaces. If this isn't whacky enough for you, the theory also claimed that 'mole people' inhabited the empty spaces within the Earth.
If Adams had simply believed this lunacy in the privacy of his Bay State home, it might have been overlooked. But he was so convinced of this theory that he sought taxpayer funding to send an expedition to 'Middle Earth' to find out the truth. Amazingly, this was actually a platform in his (successful) 1824 presidential campaign. And, a proposal to fund such an expedition came to a vote in the U.S. House of Representatives—but was thankfully defeated by a vote of 56-44.
Still, one of our most brilliant and accomplished Presidents held a lifelong belief in inhabitants of a Hollow Earth. Makes you wonder!