The immediate giveaway, perhaps, was the attached document proving the sanity of the sender, an American eccentric named John Symmes.
"I declare the earth is hollow," Symmes began, "and habitable within; containing a number of solid concentrick [sic] spheres, one within the other, and that it is open at the poles 12 or 16 degrees; I pledge my life in support of this truth, and am ready to explore the hollow, if the world will support and aid me in the undertaking."
He added with aplomb: "I ask one hundred brave companions, well equipped, to start from Siberia in the fall season, with Reindeer and slays [sic], on the ice of the frozen sea: I engage we find a warm and rich land, stocked with thrifty vegetables and animals if not men, on reaching one degree north-ward of latitude 62; we will return in the succeeding spring."
There were no takers. But Symmes would eventually earn an enormous audience for his theory in the US, touring tirelessly in the 1820s. He was largely ridiculed, sure, but miraculously managed to get a congressman to petition his colleagues in Washington for the funding to reach the North Pole and discover the 4,000-mile-wide entrance to the "hollow."