why do some rivers flow underwater or underground

It's pretty wild when you think about it, but nature has a way of hiding things in plain sight. In my travels, I've seen some weird stuff underwater, but the underground river thing is mostly about the rock layers. Usually, it happens because the ground is made of stuff like limestone that dissolves easily when water hits it. Over time, that water carves out massive tunnels and caves beneath the surface, so the river just ends up taking the path of least resistance through those hidden channels.

As for the underwater rivers, that's a whole different beast. Down in the deep ocean, you sometimes get these currents that act like rivers because of density differences. Basically, when water has a lot of salt or is super cold, it gets heavy and sinks, creating a sort of stream that flows along the sea floor underneath the normal water. It's honestly crazy to look at the sensors sometimes and see what looks like a river flowing through the middle of the ocean depths. It's just how the physics of the planet works out, keeping everything moving around.