Water and lava flowings

Everytime if i start a new world, I straightly made a strip mine to look for caves and then i found one.
When I was exploring, this strange water flow happened.......(shown below)
And Im very confused because I cant see anything when im swimming there. And also there are invisible lava pools.. yes INVISIBLE! (sorry, I didnt take a photo of it because i died...)
And thats it.. this always happens until I stop turning on caves and cliffs pack ever AGAIN