In painting backdrops for the dioramas at the Denver Museum of Nature and Science in the 1970s, artist Kent Pendleton hid eight elves. “It was just kind of my own little private joke,” he said in 2018. “The first one was so small that hardly anyone could see it, but it sort of escalated over time, I guess. Some of the museum volunteers picked up on it and it developed a life of its own.”

The museum’s field guide currently lists nine hidden finds, but there are more — the exact number is not known.