There is a
school of thought out there, propounded in the book
Fingerprints of the Gods, that believes that the
Great Pyramid and its partners on the
Giza plain were built by a pre-Eqyptian civilization that was wiped out by the cataclysm at the end of the last
Ice Age about 10,000 years ago. In this theory, the ancient
Eqyptians are believed to have renovated, not built, the pyramids and recarved the head of the
Sphinx from a lion to a man. For example, the body of the Sphinx (but not the head) shows heavy water
erosion, impossible unless it was built (actually, it wasn't built, it was carved out of the living bedrock) prior to the last time
Egypt was wet, over 12,000 years ago.
The argument used is that the pyramids are too well made (we couldn't make them today with similar materials to as great a level of accuracy even if we wanted) and incorporate mathematical constants (like Pi and Phi), and too many space- and earth-related proportions, alignments, and measurements to be accidental. For example, the famous King's Chamber (which is strangely bare of any heiroglyphs or markings for something supposedly made by the ancient Egyptians) is a featureless "golden" rectangular room made of huge slabs (50 tons each) of granite, with a smaller granite box (made to such exacting accuracy for such a hard material we couldn't even make it with existing tools) that is built exactly to the same proportions, with the additional bonus that the volume of the hollow is in perfect ratio to the outside volume of the box. It is obviously a monument to the race that made it, not a crypt. No mummy or treasure was found when the King's Chamber was opened for the first time in recorded history. The various mathematical clues all point to an extremely sophisticated civilization of master engineers with phenomenal tools.
The evidence of the pyramids is usually used to bolster the arguments of the "Chariots of the Gods" theory, propounded in books such as Eric Von Daniken's. A previous civilization makes much more sense than extraterrestrials if you believe that the Pyramids are too difficult a structure for the Egyptians to have created. Another piece of evidence to support this theory is the Inventory Stela, disregarded by Egyptologists because they cannot reconcile the information on it that unequivocally states that the Sphinx existed long before the reign of the Pharaoh who supposedly built it.
We are only flies on the face of history. Civilization may be wiped out at any time, through man-made or natural disasters, or random cosmic events. We all have the desire to project our society as far into the future as possible, but the millstones of history grind slow and hard. The Pyramid builders and the carvers of the Sphinx created cultural statements in stone that have lasted for millenia.
Spacecraft as Pyamid
We too have created artifacts of great complexity and cunning. They are called spacecraft. The lunar surface is littered with devices that may outlive our culture. For example, future civilizations (whatever their nature) will marvel at the remains of Apollo 11. It is made to incredibly tight tolerances out of difficult-to-manufacture materials, and is part of a transport system that managed to take its occupants and cargo sucessfully to the moon and back. Due to the airlessness of the Moon, it will probably outlast the Pyramids. Any future civilization that advances far enough to go to the moon and find the devices there will appreciate that a technically-advanced people made them and brought them to that place.