India, May 3 -- If you're watching the full moon set behind the Taj Mahal, you're likely looking at two endangered heritage sites in one frame.

The Sea of Tranquility on the moon recently became the first off-world site to make it to the list of most-endangered monuments, released every two years by the American NGO World Monuments Fund (WMF). The Taj Mahal, incidentally, was on the first such list, in 1996.

Over nearly three decades, WMF has highlighted heritage sites at risk from factors such as excessive tourism, conflicts and wars, natural disasters, and climate change. The first list featured 100 sites, drawn from public nominations that were then vetted by an expert panel. The last list before this year's featured 25 sites, includ...