Hamas “used more than 6,000 tons of concrete and 1,800 tons of metal to build hundreds of miles of underground infrastructure.” While the existence of what Israelis refer to as the “Gaza Metro,” which Palestinians call “Lower Gaza,” has been well known about for years, with Hamas leaders even boasting about it, the question remains how, in one of the world’s most poverty-stricken territories, which relies largely on aid from U.N. agencies, regional and Western powers, the terror group had the financial means to invest in such an intricate and expansive terror tunnel network. As the governing body in Gaza,...