The Ark in the movie was the Ark of the Covenant (the gilded gopher wood box built by Moses on the instructions of God); as such it contained the fragments of the original tablets of the Law, a jar...
He traveled to a city called Tanis that was buried under sand during a sandstorm, which is why it looks like the Nazis are digging in the desert
USA PG/Canada 14A
June 12, 1981.
Steven Spielberg.