On the fourth night of Expedition Week (Thursday), Nat Geo takes you in to the heart of the Amazon to uncover the lost city of El Dorado...

• Some authorities believe that the river was named after the Amazons, women warriors of Greek mythology who were thought to reside in the region. Other scholars insist that the name is derived from the Native American word amassona, meaning "boat destroyer."
• The total length of the river--measured from the headwaters of the Ucayali-Apurímac river system in Peru--is about 4,000 miles (6,400 kilometres), which is slightly shorter than the Nile River but still the equivalent of the distance from New York City to Rome.
• It is estimated that about one-fifth of all the water that runs off the Earth's surface is carried by the Amazon.
• The flood-stage discharge at the river's mouth is four times that of the Congo and more than 10 times the amount carried by the Mississippi River.
• The Amazon Forest holds some 2,500 tree species and 30,000 plant species (30 percent of all plant species.
• The Amazon's largest animal is the manatee, which can weigh half a ton and measure almost 10 feet in length.
• The Amazon anaconda can reach 33 feet in length.