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| Segment | Start* | End** | Length* |
| Introduction | "On the night of June 25, 1872 ..." | "Tunnels, coming next on Building Big." | 2:44 |
| Big Dig: problem-solving; tradeoffs | (2:44) Traffic on bridge | "... just as they once did in the greatest city of the ancient world." | 2:01 |
| Roman Tunnels: aqueducts; gravity; problem-solving | (4:45) Model of Rome | "... one of Roman engineering's most fundamental designs." | 5:12 |
| Canal Tunnels: arches; side support; materials | (9:57) Paw Paw tunnel | "... lost patience with such a slow means of transportation." | 4:35 |
| Hoosac Tunnel: problem-solving; explosives; surveying; measuring; worker safety | (14:32) Exiting canal tunnel | "... building even longer tunnels in a fraction of the time." | 7:56 |
| Soft-Ground Tunnels: Brunel family; problem-solving; tunneling shield | (22:28) Railroad tracks | "... engineers would have to find a better way." | 9:59 |
| Car Tunnels: subways; ventilation; problem-solving; fire | (32:27) London street traffic | "... need the best possible escape routes." | 7:53 |
| The Chunnel: symbolism; safety; tunnel boring machines | (40:20) English Channel | "... last uncharted frontier: the underground." | 12:40 |
The need to cross a river, mountain, city, or ocean channel is sometimes great enough to justify the danger, difficulty, and expense of building a tunnel. The program describes the methods and machines involved in burrowing through the solid rock of a hillside or the soft mud beneath a river.