PARIS - My visit to the Musée d'Art Moderne de la ville de Paris yesterday reminded me of another museum with not one but two unsolved thefts.

The Montreal Museum of Fine Arts was robbed in 1972 and had 18 paintings stolen by three thieves who have never been convicted and the paintings have not been seen since the thieves failed to show up to collect a ransom for the kidnapped paintings (you can read about Canada's largest theft here).

In October 2011, a man walked out of the museum in Montreal with two objects from antiquity.

Can you think of other unsolved museum thefts?

How about the 1990 theft of Boston's Isabella Stewart Gardner Museum?

Those are the mysteries that bother me the most -- what are your most problematic museum thefts?