A Metal Block is a large block made of metal and is the strongest block in the whole game. You can only break it with the Ultra Hammer. It is mostly found in Lavalava Island and Toad Town Tunnels.
In Paper Mario: The Thousand-Year Door, a couple of Metal Blocks can be found in the Riddle Tower in the Palace of Shadow. Here, they cannot be destroyed, even with the Ultra Hammer; instead, Mario must solve a puzzle to eliminate them.