The Great Pyramids, located at Giza on the west bank of the Nile River north of Cairo, are the only wonder of the ancient world that has survived to the present day. The three pyramids–Khufu (Cheops), Khafra (Chephren) and Menkaura (Mycerimus)–were built between 2700 B.C. and 2500 B.C. as royal tombs. The largest and most impressive is Khufu, which covers 13 acres and is believed to contain more than 2 million stone blocks that weigh from two to 30 tons each. For more than 4,000 years, Khufu reigned as the tallest building in the world. In fact, it took modern man until the 19th century to build a taller structure. Amazingly, the nearly symmetrical pyramids were built without the aid of modern tools or surveying equipment. Scientists believe that the Egyptians used log rollers and sledges to move the stones into place. The sloped walls, which were intended to mimic the rays of Ra, the sun god, were originally built as steps, and then filled in with limestone. The interior of the pyramids included narrow corridors and hidden chambers in an unsuccessful attempt to foil grave robbers. Although modern archeologists have found some great treasures among the ruins, they believe most of what the pyramids once contained was looted within 250 years of their completion.