SOUTH OF BAGHDAD — In a valley sculpted by man, between the palms and roses, lies a vast marble and steel city known as Al-Tuwaitha.

In the suburbs about 18 miles south of the capital's suburbs, this city comprises nearly 100 buildings — workshops, laboratories, cooling towers, nuclear reactors, libraries and barracks — that belong to the Iraqi Atomic Energy Commission.

Investigators Tuesday discovered that Al-Tuwaitha hides another city. This underground nexus of labs, warehouses, and bomb-proof offices was hidden from the public and, perhaps, International Atomic Energy Agency inspectors who combed the site just two months ago, until the U.S. Marine Corps Combat Engineers discovered it three days ago.

Today, the Marines hold it against enemy counter-attacks.