In reply to jhsp:
They are probably natural. Some species of barnacle extract excess iron from seawater as a by-product of their filter feeding and deposit it in their oddly shaped outer shell (really a modified carapace). When they die, the rest of the shell decays leaving the rusty iron deposits and a charactistic hole where the original opening was.
They occasionally appear in all sorts of odd coastal locations. It causes no end of misunderstandings.