I read this book when I was very young, so it might have been published in the 60's or 70's. Definitely not 80's or later. I'm not sure whether it was a children's book or YA. I think it was about a boy who drops something, which falls through a crack in the sidewalk or the street. He somehow gets inside the crack in search of the object and follows a tunnel or a crevasse down to an underground grotto where there's a lake and a city around the lake, with weird inhabitants. I think the book was illustrated with cartoonish black and white line art.