SK Engineering and Construction Co. (SK E&C), one of South Korea's leading builders, said Monday that it has started work to bore the undersea tunnel below the Bosporus Strait. The company that secured the order in 2008 under a build-operate-transfer arrangement said a specialized 3,300 ton tunnel boring machine (TMB) has been employed to dig the 5.4-kilometer-long tunnel to link the towns of Kazlicesme and Goztepe, located across the strait from each other.