|Have you heard about the Egyptian Lighthouse (one of the seven wonders of the ancient worlds) that was found in the sea? Well, people are proposing lifting the 70-ton sections of the lighthouse with balloons. Can a balloon do this? What special aspect of the balloon will lift 70-ton bricks? What kind of balloon would be used?
I would guess that they intend to use balloons in the water. The blocks are at the bottom of the sea, so they must be lifted up to a boat. A balloon experiences an upward net force that is equal to the difference between the upward buoyant force on it and its downward weight. If the balloon displaces air, then the buoyant force on it is rather small and it would have to be extraordinarily big to displace enough air to lift a 70-ton block. But if it displaces water, then the buoyant force on it is much greater. To displace 70 tons, it would only have to displace about 65 cubic meters of water. That's not hard at all. The balloon could be made of heavy reinforced canvas and still work just fine underwater.