Arctic exploration is the physical exploration of the Arctic region of the Earth. It refers to the historical period during which mankind has explored the region north of the Arctic Circle. Historical records suggest that humankind have explored the northern extremes since 325 BC, when the ancient Greek sailor Pytheas reached a frozen sea while attempting to find a source of the metal tin. Dangerous oceans and poor weather conditions often fetter explorers attempting to reach polar regions and journeying through these perils by sight, boat, and foot has proven difficult.