This is a cool optical illusion. Apparently the photo you are seeing came from a set of pictures provided by the owner of a house to some people who were looking to rent it for the weekend. They saw this picture and thought it was a lake, so were all excited and looking forward to boating and jet skiing and stuff. When they asked the owner about it, the lady emailed them back and said: “unfortunately that is not a lake, it is a wall“.

Here it is with some embellishments to help you decide what you’re looking at: