Daniel Terdiman/CNET

PARIS--If the Eiffel Tower makes you think of a direction, I'm willing to bet it's up. But for some people, the iconic French landmark can also mean down.

Down, as into a "secret" military bunker that has an entrance just feet from the south pillar, or leg, of the tower, and which then goes underground and which is full of Eiffel Tower history and even legend.

Secret of course, is a marketing term when it comes to this bunker, since it is open for a small number of weekly public visits. But because it belonged to the French military, it comes with a heady dose of intrigue. It's said even now that there is a tunnel that stretches from the bunker all the way to the French Ecole Militaire--military academy--at the other end of the famous Champ de Mars park. But, we're told, maybe no one outside the army really knows if that's true.

I've come here on Road Trip 2011 for a behind-the-scenes tour of the Eiffel Tower. And we've started below ground since what better way to grab someone's attention at one of the tallest attractions in Europe than to lose altitude.

Whether there's a lengthy hidden tunnel from there, there's more than one surprise to be found: a passageway with a ghost that echoes shouts on command. The ones yelled in the direction of a small gate that disappears into oblivion, at least.

What there really is here is a collection of wonderful photographs of the tower's earliest days, as well as a pictorial recounting of some of its most important milestones. This is a tower that was built for the World's Fair of 1889--in just two years, two months, five days--and which was originally slated for just a 20-year run dominating Paris' skyline.

But the tower's designer and spiritual guide, Gustave Eiffel, had other ideas. He knew that if he could bestow upon his baby some crucial attributes, it might survive. And so the Eiffel Tower became home to some of the earliest radio transmissions, as well as to the most cutting-edge experiments in meteorology, astronomy, and the monitoring of "physical and air resistance phenomena."

And, as you may have guessed, he saved the tower. Today it is home to a nest of antennae, and over time it has been the source of broadcasts like that of the coronation of Queen Elizabeth II of England in 1953.

Hydraulic elevators

Another side of the Eiffel Tower that isn't usually on people's radar is the engine room that runs its original hydraulic elevators. A relic of the tower's nascent days, these are nevertheless still a fully functional part of the every day operations. And again hidden from the view of the general public, treasures await historical infrastructure geeks: beautiful, archaic systems that are said to be much more efficient than the modern-day electric elevators that share the duty of bringing visitors up from the ground.

The doors to the hydraulic elevators first opened in 1899. According to an official Eiffel Tower accounting, the yellow "chariots,"

"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."

What this really means is that down here, behind the walls, and at the base of the great elevators that take guests to the 377-foot-high second level, there is some beautiful, very old infrastructure. And though it's the electric elevators that take people up to the 905-foot-high third level--with all the antennae bringing the tower's total height to 1,063 feet--those are the less reliable. The 100-year-old-plus systems are the ones that break down less.

Going up the tower

Going to the top of the Eiffel Tower is something that's not for everyone. If you go as high as you can go, you're exposed to the elements--though protected by a level of fencing that keeps everyone in--and it's a long, long way down.

When I was a kid, I went to the top and had to hug the wall to keep from freaking out completely. These days, I'm a bit better at taking heights, and I actually enjoyed standing at the edge and looking at the tiny people, cars, buses, houses, and so on, far below on the streets of Paris.

But maybe it was just easier because this time around, there's a functioning Champagne bar on the third level. Bottom's up.