Swimming pool under floorboards

Hi all, hoping you can shed some light. I’ve just bought my very first house and started working on it this Bank Holiday weekend.

I’m changing the carpet so l was just securing some of the floorboards in the lounge - I was a bit nosy and wanted to see what’s underneath and to my horror, there’s a pond down there!

The stagnant water is around 1.5 inches and goes back a good 2 metres. What the heck should I do? How big of a concern is this? This is my first house so I’m absolutely clueless.