Basements are archetypal symbols of the unconscious. In dreams, for example, houses nearly always symbolize the mind, and the basement of the house is the unconscious part of the mind. That makes basements mysterious and spooky on a level which transcends any kind of rational thought. It’s unsurprising that so many horror movies take place in basements and cellars.
I’d like to add that when I was young, my brother and I experienced some very disturbing things in our basement, which is where the television set and rec room were. We would often hear thumps from the storage area, followed by waves of cold. Once, both of us simultaneously got feelings of terror for no reason we could explain—and then a clock simply flew right off the wall. And on another occasion, the extension cord to our television set began spraying a huge, bright fountain of yellow sparks, like an angle grinder working on a piece of metal. We stared at it incredulously as it just kept going, lighting up the whole room with its almost cheerful brightness. We charged upstairs to tell our parents, but by the time they got downstairs it had stopped, without leaving any sign of what had happened.
I should add that our basement also repeatedly flooded. After the second time the basement flooded, the landlord paid to have the entire water main dug up and replaced… after which it promptly flooded again.
I don’t believe in “the supernatural,” but I have to admit that I don’t have an explanation for what we experienced except that, as I noted, basements are powerful archetypal symbols, and we may have been having a sort of joint hallucination through the collective unconscious.