Beach visitors for years have wondered why rubber-like slabs, imprinted with the word "Tjipetir," were washing up on beaches throughout northern Europe.

But recently,the history of these blocks was uncoveredby British beachcomberTracy Williams. And it dates all the way back to submarine warfare of World War I.

Im not a historian, I just clean the beaches, she says. I do walk on the beach with my dog every day, and Im always absolutely intrigued by everything that washes up and where it comes from.

Back in 2012, she discovered her first Tjipetirblock during a beachside stroll inCornwall, England.

I took it home, and I googled the word 'Tjipetir,'and at that time there was hardly anything on the internet about it.It really just said it was a village in Indonesia, she says. So, she put it in her yard and forgot about it.

But just a few weekslater, on a different beach,she found the sameoddity this time, accompanied by bales of rubber.Finding one was understandable, but a second one turning up was quite odd.

Naturally, Williams got curious and decided to do some investigating.

Post by Tjipetir Mystery.

Turns out, Tjipetirwas the siteof a 19th century Indonesian rubber plantation. She also discovered the slabs aren't technically made ofrubber, but rather, most likely, something called gutta-percha, a substance that is yielded from a tree native to Malaysia and the Malay Peninsula. Because gutta-percha predates plastic, it was used for everything from teddy bears' noses to book bindings, hot air balloons,firemans hoses,and most commonly waterproof coating for submarine cables.

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We finally know why these mysterious 'Tjipetir blocks' are washing up on European beaches