Henry Sinclair was an Earl of Orkney, Scotland, in the late 14th century. Half a century ago he was theorized to have explored North America before Columbus. Sir James Gunn may have accompanied him, and may have died in the area now known as Westford, MA. While Pre-Columbian exploration is no myth, and not all that remarkable when Native Americans were here long before, Henry Sinclair’s journey is highly debatable.
Here is a link to Archivist Brian Smith’s discussion of the subject: