- [Narrator] After months of preparation, the tower's construction can finally begin.

But being so close to the river means the soil is soaked with water.

How to build the foundation for the world's tallest building on such potentially unstable terrain?

In the middle of the Paris winter, the work begins.

Soon, around 500 workers gather to dig the foundation of the tower's north and west pillars, the nearest to the Seine.

Piece by piece, just like in Bordeaux, large watertight metal boxes are assembled, which will form the pressurized chambers, or caissons, to allow construction of the foundation to be protected from flooding.

Then the project encounters a serious problem.

- When the pressurized air chambers come into use, workers develop an unknown illness.

They report tingling sensations, bleeding, difficulty breathing, and partial paralysis.

No one understands the cause of this ailment nor the importance of making decompression stops when coming back to the surface.

(gentle suspenseful music) - [Narrator] Today, the illness is known as the bends or Caisson disease.

Inside the caisson, much like underwater divers, workers breathe air at a high pressure.

(gentle suspenseful music) But if they return to the surface too quickly and the pressure drops rapidly as a result, nitrogen bubbles can form in their blood, causing decompression sickness.

(gentle suspenseful music) Nobody understands what is happening, even the government is concerned about the potential danger.

- In April 1887, the Minister of Commerce and Industry decides to go down himself into the foundations, and he comes back up alive.

- It was a demonstration that the caisson was not so harmful and it was not a problem, which could delay the construction of the tower.

(suspenseful music) - [Narrator] Despite the discomfort, work resumes until the piers are in place.

Soon, solid masonry rises from the foundations to support the metal structures at the bottom of the tower.

Now they can begin the iron work.

(iron clanking) (suspenseful music)