The Eiffel Tower, behind the scenes (photos)

PARIS--Completed in 1889, the Eiffel Tower is without question one of the most famous landmarks on the planet. It was built over the course of two years, two months, and five days for the World's Fair of 1889.

One of Gustave Eiffel's greatest achievements with the tower was the use of hydraulic elevators, an innovation that enabled hundreds of thousands of people to easily ride 380 feet above the ground.

As part of Road Trip 2011, CNET reporter Daniel Terdiman visited the Eiffel Tower and got a behind-the-scenes tour that, among other things, explored the "secret" underground military bunker beneath the tower, as well as the inner workings of the hydraulic elevators.

Here, we see the Eiffel Tower from just in front of it.

Some might consider the hydraulic elevator system that Gustave Eiffel designed in 1889 to be well ahead of its time. Here's how it works, according to information provided by the organization that runs The Eiffel Tower:

The elevator "passenger compartments, mounted on a carriage and kept horizontal by a leveling system, are pulled upwards by cables that move in line with two parallel pistons located underground, via a cable drum system--the cables themselves, the ends of which are attached to the passenger compartment carriage--run back and forth eight times over two sets of pulleys, one of which is fixed and the other attached to the moving pistons, thereby ensuring that the [elevator] passenger compartments can travel [420 feet], i.e. eight times the piston travel--of 52 feet.

"The pistons are actuated by a water circuit with a pressure of 40 to 60 bar, which until 1986 generated motion thanks to three large accumulators of some 200 metric tons each, which provided both the pressurized water reserve--the energy to drive the motion--and the counterweight function.

"Since modernization in 1986, high-pressure, oil-driven hydraulic motors drive piston carrier motion while two of the three accumulators serve as counterweights."

Here we see two of the hydraulic lifts and one of the accumulators.

These big red wheels are one of the sets of pulleys mentioned in the previous captions. These are the pulleys attached to the moving pistons.

Just adjacent to the south pillar of the Eiffel Tower, there is a nondescript green cage. This is, in fact, the entrance to a "secret" underground bunker, which was originally designed to belong to the French military. Although today the bunker is open to small guided public groups and contains a collection of Eiffel Tower memorabilia, posters, and pictures, it still connects to military tunnels, and the public is only allowed to go a very short distance inside. It's not known how far or where the tunnels go, but legend has it that they stretch all the way from the Eiffel Tower, under the famous Champ de Mars, and to the Ecole Militaire, the major French military academy.

Here, a small public group heads down into the "secret" bunker.

This diagram illustrates the principles of the hydraulic elevator, as explained in the second caption. Today, two of the elevators are still hydraulic, but two are electric and were installed by Gustave Eiffel for the World's Fair of 1900. But the electric elevators have proved to be more fragile and require more maintenance. Two hydraulic elevators remain to this day as a cultural reminder of the origins of the tower.

This is a view of the very top of the Eiffel Tower, an antenna that rises above everything else. Visitors can reach the third level of the tower but still must look up at a group of antennas and other telecommunications equipment.

Here, we see a wide-angle view of the underside of the Eiffel Tower's second level, as seen from the first level.

Here we see one of the tower's hydraulic elevators, just adjacent to one of the famed yellow accumulators that helps drive the system that brings the elevators up and takes them down again.

This chart, on the wall inside the main administration office for the Eiffel Tower, shows the tower's growth during its construction period from 1887 to 1889.

Here, we have a view up through one of the legs (pillars) of the Eiffel Tower.

Here, you can see one of the spiral staircases that wind up--or down--through the legs (pillars) of the Eiffel Tower.

This is a view down at the ground underneath the Eiffel Tower from its first level. You can see the characteristic arches of the lower parts of the tower, as well as its iconic shadow.

In this photograph, taken from the third level of the Eiffel Tower, you can see the very tip of the tower and many of the antennas and cell phone repeaters that are located at the top.

Here and there around the Eiffel Tower are mannequins of workers who helped build or maintain the famous monument.

This is a bust of Gustave Eiffel, located just adjacent to the tower's administrative offices in the north pillar.

Just in case anyone needed a reminder, this sign, located in a hard-to-see place, reminds people that climbing the Eiffel Tower is not allowed.

This is a photograph of a historical picture of the Eiffel Tower showing what are called the foundation caissons, essentially waterproof metal cages that made it possible to construct the tower's foundation adjacent to the famous Seine River of Paris.

This is another mannequin, this time of a worker hanging upside down while performing a task of some sort.

This is an old spiral staircase, different than the exposed staircase that visitors can now use to climb up or down.

This is one of the electric elevators, seen here inside one of the pillars of the Eiffel Tower.

On the third level of the Eiffel Tower, there are markers showing the distances from the tower to famous cities around the world in the direction you're looking. Here, we see the sign showing the distance to New York City, Washington, D.C., and (more roughly) to Mexico.

This is a junction of structure struts, very high up on the Eiffel Tower, with the Seine River visible far below.

For many of those who have gone up the Eiffel Tower, one memorable sight is that of one of Paris' metros crossing the Seine on the bridge near the tower.

For those who don't bring their own to toast with, there is a champagne bar located on the third--highest--level of the Eiffel Tower.

There is an apartment on the third--highest--level of the tower, and those who look in the windows will see mannequins of Gustave Eiffel (right) and inventor Thomas Edison. The two were friendly, and Edison visited the apartment, which Eiffel kept for entertaining illustrious visitors, on September 10, 1889.

This is a view of Paris' Palais Chaillot in Trocadero, one of its famous monuments, across the Seine River from the Eiffel Tower.

Here we see a view of the famous Champ de Mars, which connects the Eiffel Tower with the Ecole Militaire, from inside the structure of the tower.

Here we see a view looking straight down one of the elevator tracks of the Eiffel Tower.

Here, we see another mannequin; this one is mounted just below one of the hydraulic elevators and represents an early driver of the elevator.

This is a view to the top of the famous Eiffel Tower from the second level.

Here we see one of the hydraulic elevators inside the structure of the tower.

This is a passageway inside the military bunker that is said to have ghosts behind it.

According to a plaque in the military bunker, "Gustave Eiffel knew that in order to survive, the Eiffel Tower would have to prove its practical worth for science. The tower was scheduled to be dismantled after just 20 years. [So] he...allowed it to be used for a wide range of scientific experiments, from meteorological and astronomical observations to monitoring physical and air resistance phenomena.

"He secured the future of his tower with the installation of a huge wireless transmission antenna." The tower then became the site of the first radiotelegraph broadcasts of the early 20th century, the plaque says.

In 1935, the Eiffel Tower transmitter became active, and on June 2, 1953, English Queen Elizabeth II's coronation was broadcast live throughout France from the Eiffel Tower.