I believe that one of the biggest frustrations for unesco collectors is to visit a site or beeing close to that site and couln't find any cards of it. That's pretty much what happened to me in Gibraltar. I didn't visit these caves but I visited every souvenir shop hoping to find cards but had no luck. 3 years after my visit to the Rock, I finally have a card of the caves, sent from Gibraltar and Eric even used a matching stamp.
Photo by K. Martinez
The steep limestone cliffs on the eastern side of the Rock of Gibraltar contain four caves with archaeological and paleontological deposits that provide evidence of Neanderthal occupation over a span of more than 100,000 years. This exceptional testimony to the cultural traditions of the Neanderthals is seen notably in evidence of the hunting of birds and marine animals for food, the use of feathers for ornamentation and the presence of abstract rock engravings. Scientific research on these sites has already contributed substantially to debates about Neanderthal and human evolution. - in: https://whc.unesco.org/en/list/1500/