Take the largest warship/aircraft carrier ever built that is 872 ft. long and displaces 73,000 metric tons, and keep it hidden while it's being built until its launched. That's exactly what the IJN Navy accomplished in 1944. How did Japan accomplish this? The Japanese workers were loyal to their Emperor and their country (they would also be killed if the ships existence was mentioned), and the ship was surrounded by tall fences in a graving dock.
It was only photographed twice in its short lived existence. It was sunk only 10 days after it was commissioned. The submarine USS Archerfish sighted it at 20:48 after it had departed Yokosuka and was traveling to naval base at Kure for being fitted out. Six torpedoes ultimately sank the marvel of mankind's engineering. But, the US Naval command was not sure of what exactly USS Archerfish had sunk, until the war was over.
Now that's a well kept secret! However, the accidental stumbling of the USS Archerfish onto the path of the Shinano and its escorts sounds too good to be true. Did the US Naval Command in the Pacific have local intelligence? What do you think ?
The Shinano at sea after being launched from Yokosuka.
The USS Archerfish photographed in 1945, few months after sinking of the Shinano.