BIG turtle

As we were getting ready to leave our spearfishing spot I saw what I thought was a big flat rock under the water. The rock started moving and it turned out to be one of the biggest sea turtles I've ever seen. It's shell was about the size of a refrigerator door and the head was as big as a half gallon milk carton. I wonder how old it is? No one know's what their life span is but they take up to 4o years to mature into adults.