I found this article today and thought this was a fascinating piece. An excerpt:

NEW HAVEN, Conn. -- As art restorers in London inspected a 230-year-old painting by master landscape artist Thomas Hodges, they noticed the canvas was thicker in some areas than others. Using an X-ray machine, they peered behind the lush greens of New Zealand and discovered the oldest known painting of Antarctica.

Just the other day I noticed that here, in my home office, I have a few mottled spots of paint which seem raised higher than others. I had it X-rayed, but nothing conclusive showed up. One art expert, foremost in his field, believes it's an original but accidental hand-print left in plaster by one of the finishers back when my condo was built in 1978.

Monty

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The painting which apparently, under X-ray, reveals secret pirate booty buried under an Antarctic penguin nest.