Leaning Tower of Pisa (03:26)

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The tower is in danger of collapse. In 1995, Giorgio Macchi attempts to stabilize the building, but the tower begins to move. Credits)

Rescuing the Leaning Tower of Pisa (03:10)

A 14th century bell tower collapses, killing four bystanders in Pavia. John Burland investigates the soil and Macchi examines the masonry for structural problems; the tower closes to visitors for three months.

Structural Issues (05:01)

Flaws in the Leaning Tower of Pisa include the shape of the tower, wall thickness, and composition. Macchi examines the marble's stress. Plastic-coated tendons provide support, but the lean continues to increase.

Italian Architecture (03:04)

Medieval Pisa is a wealthy, bustling trade town. Work on the cathedral begins in 1064; it houses a mosaic of Jesus Christ in the apse. The architect of the Leaning Tower of Pisa does not leave a signature.

Tower Builders (05:19)

Jane Morley believes the workers likely had no formal education. After reaching the fourth level, work stops for nearly a century; the ground strengthens. Carlo Viggiani describes how the masons constructed the bell tower to correct for the lean.

Examining the Tower (04:07)

A surveyor takes readings twice a day and brackets cracks. Infrared cameras reveal that the building sways in a circle every day; rainstorms cause large movements.

Sinking Tower (05:47)

Scientists reconstruct Pisa's geographic history in Bergamo using a model and centrifuge. John Burland explains why propping up the tower would cause collapse. Engineers incorporate lead ingots on the north side for support.

Leaning Tower of Pisa: Long-Term Plan (03:55)

Burland hypothesizes that soil extraction might be the most efficacious strategy to save the tower. Experts cannot reach an agreement.

Failed Interventions (03:53)

In 1838, Gherardesca digs a walkway around the tower to reveal the base of the columns. Engineers install a plumb line and drill holes into the foundation, causing destabilization. The bells lock because of vibrations.

Black September (05:11)

Engineers propose attaching anchors and securing them in the bedrock. Workers inject liquid nitrogen into the soil. Workers add 300 tons of lead to stabilize the building.

Committee Setback (04:23)

The committee disbands after a dispute within the Italian government. A strong earthquake occurs and the group reconvenes, approving soil extraction. Workers install a temporary harness to protect the building.

Successful Intervention (03:32)

Burland analyzes the soil and discovers it is silt. Engineers worry that drilling will cause the tower to lean more. It will take two years for the tower to return to its original form.

Credits: The Riddle of the Leaning Tower of Pisa (00:40)

Credits: The Riddle of the Leaning Tower of Pisa