https://www.nbcnews.com/news/world/terrifying-plastic-rocks-found-remote-brazilian-island-rcna75217
Allegedly, this has been discovered in 2006, but I clearly remember seeing such rocks on shores of Adriatic in middle of 1990s and I'm pretty sure they have been around for quite some time. It doesn't take a long time to form in conditions of hard water and broken karst.
All in all, it is a sad lithospheric reminder of anthropocene.