Back in the 1970s, the Denver Museum of Nature and Science hired artist Kent Pendleton to paint the backdrops for many of the museum’s wildlife dioramas. Little did it know that Pendleton’s penchant for hiding tiny mythical creatures in these paintings would add a whole new dimension to the museum experience.

It all began with eight elves—or gnomes, or leprechauns, depending who you ask—hidden in Pendleton’s wildlife dioramas. An elf hiding in the lowland river. An elf riding a dinosaur along a cretaceous creekbed. Another elf sat on a rock in the Great Smoky Mountains. And others, hard to spot but definitely there, in various backdrops throughout the museum.

When these eagle-eyed volunteers began to spot the museum’s incongruous and thoroughly unscientific inhabitants, the whole thing began to snowball. The staff decided to go along with the game, adding more elves and gnomes to the museum. A ceramic elf, for example, found his way onto the Candor Chasma of Mars. And now a digital elf exists in the entrance video, cleverly concealed within a cluster of stars.

The fantasy easter eggs diversified, too; there are angels, unicorns, even a Millennium Falcon and a tiny Yoda hidden in the museum. Precisely how many creatures are hidden around the museum is an open question. The museum’s official elf scavenger-hunt guide currently lists nine. But Maura O’Neal, the museum’s communications and media relations manager, says there are about double that amount.

So even if you do go on the scavenger hunt, guide in hand, you’ll never quite know when you might spot an undocumented elf lurking somewhere, surreptitiously, in the Denver museum…