What is the geological history of Ha Long Bay?

Ha Long Bay Geology: History & Formation?

Okay, so like, Ha Long Bay? Beautiful place. Went there on 12/03/2018, cost me about $500 for a 3-day tour from Hanoi. Stunning.

Geologically speaking, the bay's super old. We're talking way back, like 500 million years before anything I ate for lunch. Back then, just part of the ocean, y'know?

But then - hold on - this is where my brain gets a lil' foggy. The tour guide, she was saying something about Carboniferous and Permian periods. Basically, 340-250 million years ago (give or take), which is just, wow.

Then get this: something MAJOR happend. Tectonic plates and stuff shifted like a grumpy cat, making the area shallow. Up it went from deep sea! That's why the karst formations are there, I think, due to erosion over millions of years (I’m not geologist, lol).