The Great Pyramid of Cheops

Believed to have been erected about 2600 BC during the reign of Khufu (Cheops), it is the only one of the seven wonders still standing. The base covers over 13 acres, it's volume is about 90 million cubic feet, and it weighs about 6 million tons. The 2 1/2 million limestone blocks that make up the pyramid weigh from 2 to 70 tons each. It is 454 feet high, about the same as a modern 48 story building. Originally, it had some 144,000 15 ton limestone casing stones on all sides giving it a mirror-smooth finish. They were polished to within 1/100 of an inch, an extraordinary degree of precision typical of the entire structure.

The casing stones were lost to earthquakes and theft over the centuries, leaving the 203 courses, or steps, bare. Only a few casing stones remain in position. Unlike other Egyptian pyramids, it has a flat top of about 30 square feet -- it lacks an apex stone for some unknown reason. It is aligned to true north with only 3/60 a degree of error by modern reckoning. Since the North Pole moves over time due to the earth's wobbly spin, the pyramid was exactly aligned at one time.

There are three major chambers in the pyramid, known as the King's chamber, Queen's chamber, and unfinished chamber connected by a system of shafts, passages, and minor chambers. A grand gallery leads to the King's chamber. The temperature inside is a constant 20°C year round, equivalent to the average temperature of the earth.

The purpose of the pyramid remains a mystery. Theories include tomb, monument to the Pharaoh, astronomical observatory, sundial, grain storage, knowledge repository, or a place for rituals.   Link.