MegaStructures - Underwater Highway (National Geographic Documentary) MegaStructures - Underwater Highway MegaStructures (National Geographic .\rbr\rbrMegastructures - Korea Engineering Underwater Highway Busan Documentary National Geographic. Largest Digging machine Worlds biggest digging .\rbr\rbrMegastructures - Korea Engineering Underwater Highway Busan Documentary National Geographic. Largest Digging machine Worlds biggest digging machine .\rbr\rbrUnderwater Highway, Busan Korea (Engineering Megastructures) Great Engineering Projects :- Under water highway project in Busan, S.Korea. Credit: History .