Recently, my father and I were cleaning out the garage at our place where we removed an old defunct heater that sat in the back of the garage for years. When we removed the heater, I noticed the wiring was too light to be copper and upon closer inspection, the sheathing said "KAISER ALUMINUM".

I've never seen Al romex before in person so I thought it was pretty neat to come across this, however, I do recall reading stories of house fires, electrical failures, etc from this type of wiring.