When I was a child, I had an interesting chat with an old smelly woman not many other people could see. She wasn't imaginary, just not many people could see her.

Also, when I was twelve, me, my cousins and my friends used to like to go adventuring in the sewage pipes. The last time we went, we saw a glowing humanite sort of figure standing on the corner of a tunnel intersection, just like it was peering around the corner at us. We were kids and had to bend to fit in the pipes, but it looked like this thing was standing up straight. The tunnels are dark and reflect no light. I wasn't at the front and I just made out the figure that look to be human, my brother said he saw grey eyes on it.
Nowadays when we look back, we usually come to the conclusion that it was some bald teenager high on acid with glow on the dark stickers pasted all over his body. There's still mystery to it, though.