ON THE GULF OF MEXICO – Underwater robots positioned a giant 100-ton concrete-and-steel box over a blown-out well at the bottom of the Gulf of Mexico on Friday as workers prepared to drop the device to the seafloor in a first-of-its-kind attempt to stop oil gushing into the sea. A spokesman for oil giant BP PLC, which is in charge of the cleanup, said the box was suspended about 200 feet above the main leak Friday and was being moved into position, though it could be Saturday before that happens. Several undersea cameras attached to the robots were making sure...