Small treasures aren't unusual to find while beachcombing, but San Franciscans have been uncovering some surprises as of late: century-old tombstones.

On her daily walk along Ocean Beach, Teresa Trego recently stumbled across a 122-year-old granite tombstone, half buried in the sand.

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"It was right out there in the open," she told Fox News. Susan Palleschi, another San Francisco resident, also saw the gravestone on her morning jog.

"I've seen some really unusual things out here," she said to KTVU. "But that gravestone blew me away."

The incident might have been considered a random occurrence if it wasn't the second find in a month. In May, two beachgoers found a gravestone from 1876 on the same beach.

Did a cemetery wash to sea? Is this the latest development in the rumored zombie apocalypse? Did a whale swallow some tombstones during the Japan tsunami and deposit them in SF, as one Mission Mission commenter suggested?

According to a recent paper by Bill McLaughlin of the Surfrider Foundation, the historic markers are likely part of early 1900s anti-erosion efforts.

Instead of constructing another seawall, a makeshift revetment made of tombstones was dumped on the beach. The gravestones came from the Laurel Hill cemetery after if had recently closed due to pressure from developers.

According to SFBay, the remains of those buried at Laurel Hill were transported to Colma in the early 1900s to free up the expensive San Francisco real estate, but many of the tombstones were left behind. Emergency erosion crews thus braced the beach with the heavy granite stones.

"They also used them all over the city," said Western Neighborhoods Project Director David Gallagher in an interview with KTVU. They used them in the seawall in the Marina, they used them in Buena Vista Park, Noe Valley. They're all over the city. But it's kind of neat to have one just pop up."

But if the Ocean Beach revetment is eroding, this may not be the last to show up in the sand.

Watch the Fox video for more about the tombstone finds below, and visit Mission Mission for more photos: