tokens: While many sites are rudimentary , extensive workings have been found which show evidence of skillful excavation , strongly implying the assistance of tin miners , doubtless the case in the recent example of extensive excavations discovered in 2008 when builders renovating a waterfront warehouse in Penzance took up hatch covers and found several tunnels , one extending some 300 yards and emerging into the cellar of an 18th - century public house after passing beneath several roads .