A probe into the mysterious shooting of two soldiers has revealed the existence beneath the Serbian capital of a secret communist-era network of tunnels and bunkers that could have served as recent hideouts for some of the world's most-wanted war crimes suspects. The 2-square-mile complex -- dubbed a "concrete underground city" by the local media -- was built deep inside a rocky hill in a residential area of Belgrade in the 1960s on the orders of communist strongman Josip Broz Tito. Until recently its existence was known only to senior military commanders and politicians. The secret was revealed during an...