Swiss tunnel: Not actually the world's longest tunnel

When Swiss engineers completed drilling the Gotthard Base Tunnel Friday, they were lauded for finishing the world’s longest tunnel – but actually it's only the world's longest rail tunnel. Read on to find out what underground structures take the honors for being the longest.

5. Seoul Metropolitan Subway

The Seoul Metropolitan Subway's Line 5 is the longest rapid-transit line in the world right now and one of the longest tunnels, period, measuring 29.6 miles underground. The Seoul Metropolitan Subway is one of the most widely used rapid-transit systems in the world, making about 8 million trips a day between all of its lines.

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