"The Lawn Mower Repair Man's Front Page"
This is, was, the blade from a 21" walk behind Snapper mower. The air lift has been under cut by the action of sandblasting. In time the hunk of steel that was the air lift will go flying off at 200 mph. If lucky, it will be contained below the deck. But what if, what if it were to bounce out from under the deck and implant itself in the face of a five year old innocently playing next door? Not a pretty thought.
One more thing about this blade. Although it has been sharpened a number of times, it's very evident that the last time was many lawns ago. This blade stopped cutting grass a long time ago and has been beating the snot out of lawns ever since. It's easy to imagine what the lawns looked like after being thrashed by this bugger.
The engine has to work a lot harder as well when a blade is dull. Think of it this way. Try cutting a tough steak with a dull butter knife rather than a sharp steak knife. Bottom line, for reasons of safety, quality of cut, and excess engine wear, check your blade regularly.