Basements Are Scary
A couple years ago we visited my in-laws in Texas. They live in a ranch-style house with a basement that's surprisingly quite small compared to the rest of the structure, with no windows, as it's technically a storm shelter. The previous owners were workout enthusiasts, so the floor is covered with thick, springy pads and there are some big full-length mirrors, some wall-mounted fans, and plenty of lighting. Now it serves primarily as a playroom for the grandkids and a storage space for extra furniture.
My 3-year-old son couldn't reach the light switches at the top of the stairs to the basement, but he could reach the switches at the bottom, which you can safely get to if the hall light near the top of the stairs are on. Nonetheless, he wanted somebody else to turn the lights on for him, and he didn't really like being down there alone, because “it's scary down there.”
My initial reaction was to think he was being silly, but then I remembered how scared I was of my great grandmother's basement when I was little. Her basement was cramped, full of piles of old stuff, and dimly lit. My cousins and I liked to see how far in we dared to go before our imaginations got the better of us and we ran back upstairs.