There have been documented cases of them. They have been on the news, casts made, etc
The "documented" cases should have the proof available - where is it?
News reports I have heard of are generally "reported" sightings - again, where is the proof?
And if casts have been made and verified, where are they?
Instead of asking us skeptics to accept rumor and heresay, or to prove to you they do not exist, simply furnish some real proof.
With the hundreds of people whose wives cousins barbers brothers aunt that have seen one, it should be easy. Until then, I choose to remain a skeptic.