The Pantheon in Rome is a rare ancient building because it has resisted the decay of time so well. One reason it is such a treasure is that it is the only intact example of architecture of the time of that size, age and span. The ability of The Pantheon to hold up for centuries is no accident - rather, it reflects a genius of engineering skill.

The Pantheon holds up well in comparison to modern buildings for architectural innovation so much so that it has been copied often, but few have been able to match or surpass the brilliance of this ancient building. The fact that it stands to this day as a challenge to modern architects to live up to its design is only more amazing considering it was built around 125 AD, during the reign of the Roman emperor Hadrian.

The basic design is one that is brilliantly simple, placing a round enclosure next to a rectangular entryway. The gateway to the Pantheon is built in the style of a Greek portico, using columns made of granite and finished off with a triangular pediment on top. The entrance is supported by three rows of 39 foot Corinthian columns. Eight of those supports are at the front and then there are two sets further in - each with four supports that lead you onward toward the central rotunda. So the rectangular section serves the function of joining the rotunda and the portico.

This design seems amazingly simple but under scrutiny you find dozens of examples of pure genius.

For example, the massive dome of concrete that is on top of the round central component is of such elegant design and of such high quality of construction that anything less in the design of this dome would have collapsed under its own weight.

That huge dome is 142 feet across. By way of comparison, that makes it 46 feet bigger than the crowning that is on top of the White House that stands in Washington DC. The oculus of the dome at its zenith is 25 feet of that span. The unique composition, brilliant engineering and amazing construction standards are the reasons for the dome that is virtually impervious to the ravages of time.

It's easy to think that the opening in the center of that large oculus is just for artistic design. But it has an important job of reducing the weight of the dome. It is also perfectly balanced to distribute the stress of the dome's weight around its circumference. This makes the dome as resistant to being crushed as a bicycle wheel that we are familiar with.

That opening also allows light into the interior, providing a perfect natural lighting. However it also lets in the elements such as the rain so the floor below is well designed to drain off water accumulation to drains that keep it safely out from underfoot.

The talent of the designers is also on display in the tapering steps of the dome. That dome is an amazing 20 feet thick at the base of the structure and 7.5 feet thick at the oculus. The composition of the dome is also a part of the balance that keeps it aloft because the lower parts are made of heavier materials and as it rises to the peak, the material that was used is lighter. This simple design plan seems obvious but many architects down through time and to this day do not use such simple design methods to replicate the success of that dome.

It seems that the two thousand years that the Pantheon has stood has not changed its stability one bit. And if that isn't awe inspiring enough, consider that it was built without any of the modern technology, tools or machines that we use to build our buildings today.

When you admire the sturdy and elegant materials used to create the Pantheon, keep in mind also that the engineers that built this marvel had to bring in everything that went into the construction by floating those materials in on the Tiber river. From the river, those materials were laboriously moved to the site using humans and animals pulling carts.

The huge bronze doors of the Pantheon have undergone a number of restorations over the years. But there has never been any serious structural repair needed or done to the Pantheon at any time in the history of the building. That record is even more amazing considering that the Pantheon was built on marsh land.

A good comparison in terms of amazing longevity of the Pantheon is the Leaning Tower of Pisa which seems to need virtually continuous effort to support and maintain that structure. Much of the reason for the troubles of that tower is the marshy ground under the construction. Another great building that was built at the same time as the Pantheon in Rome is the Parthenon in Greece and it is for all intents and purposes a wreck 2000 years after it was built.

In the eighth century, the Roman Pantheon was adapted to be used as a church. And unlike many ancient sites, the Pantheon was not set aside just to be admired, as it has been in continuous use from the day it opened until our modern time.

Many modern architects have copied the amazing design of the Pantheon. Some famous architecture that was inspired by the Pantheon includes the Thomas Jefferson Rotunda at the University of Virginia as well as the Reading Room of the British Museum.

If you have admired those wonderful buildings, it's worth it to spend some time seeing the building that inspired them all.