What is the oldest katana in the world?

I'm curious, could you elaborate on the history and origins of the oldest katana in the world? What era was it crafted in, and what materials were used to forge such a timeless blade? Have there been any notable discoveries or exhibitions showcasing this ancient weapon, and what significance does it hold in the realm of martial arts and sword collecting? Additionally, are there any theories or legends surrounding the origins of this ancient katana that have captured the imagination of enthusiasts?