A rusty, diamond-shaped iron blade, its sharp point jutting from the dirt where it was discovered, could be a centuries-old clue that sheds new light on the obscure path taken by the Spanish explorer Hernando de Soto.

For archeologist Dennis Blanton it has erased most doubts that the patch of ground in southeast Georgia was visited more than 460 years ago by some Spanish explorers - if not by de Soto himself.

"It's pretty much case-closed," says Blanton, standing in a clearing among planted pines where his archeologists have dug about 46 centimetres into the dirt in an area the size of a small house. "If you had to deduce the most plausible source, it would be de Soto."