In this episode, we head to Boston.

The 'Big Dig' was a road and tunnel construction project designed to solve Boston's congestion problems. With the final project cost exceeding $14 billion USD, this megaproject took 20 years to complete. It involved constructing 25 km of highway lanes, 8 km of tunnel, 16 interchanges and 200 bridges.

But on the 10th of July 2006, at 11:01 pm, a section of the I-90 tunnel collapsed unexpectedly, with tragic results.

So what happened? Why did it take six years to collapse? And how could one of the most costly and complex public infrastructure projects ever undertaken in the US have failed so catastrophically?

This episode follows the National Transportation Safety Board as they answer these questions, and shares insights into how such an investigation is conducted.

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