It’s common to find, in the blank spaces of 13th and 14th century English texts, sketches and notes from medieval readers. And scattered through this marginalia is an oddly recurring scene: a brave knight in shining armor facing down a snail.

It’s a great unsolved mystery of medieval manuscripts. As Got Medieval writes, “You get these all the time in the margins of gothic manuscripts.”

And I do mean all the time. They’re everywhere! Sometimes the knight is mounted, sometimes not. Sometimes the snail is monstrous, sometimes tiny. Sometimes the snail is all the way across the page, sometimes right under the knight’s foot. Usually, the knight is drawn so that he looks worried, stunned, or shocked by his tiny foe.

Epic snail-on-knight combat showed up as often in medieval manuscripts as Kilroy across Europe. “But the ubiquity of these depictions doesn’t make them any less strange,” says the British Library, rounding up a number of examples of the slimy battles.

No one knows what, exactly, the scenes really mean. The British Library says that the scene could represent the Resurrection, or it could be a stand in for the Lombards, “a group vilified in the early middle ages for treasonous behaviour, the sin of usury, and ‘non-chivalrous comportment in general.’”

The valiant snails could be a commentary on social oppression, or it could just be medieval humor, says Got Medieval: “We’re supposed to laugh at the idea of a knight being afraid of attacking such a “heavily armored” opponent. Silly knight, it’s just a snail! “

For Digital Medievalist, Lisa Spangenberg floated another idea. She says that “the armored snail fighting the armored knight is a reminder of the inevitability of death,” a sentiment captured in Psalm 58 of the bible: “Like a snail that melteth away into slime, they shall be taken away; like a dead-born child, they shall not see the sun.”

More from Smithsonian.com:

Islam’s Medieval Underworld

Centuries Ago, a Cat Walked Across This Medieval Manuscript

The Secret Lives of Medieval Books