When manmade pollution caused Texas to be under the ocean.
https://www.smithsonianmag.com/travel/when-texas-was-bottom-sea-180953653/
The rock beneath me, which looks almost white in the glare of the sun, is full of fossils. Zillions of them. Back when these life-forms were alive—265 million years ago or so—the Guadalupe Mountains were underwater, part of a flourishing reef that once stretched about 400 miles around the edge of a long-vanished sea.