I've got myself very worried over the years at all the possible asbestos containing materials hiding in my house and now I've read about asbestos bitumen underneath tiles. I'm in the process of wondering what to do about some hollow/broken tiles and I've noticed this at the edge of some tiles at the bottom of a cupboard. It could just be some adhesive which has attracted a lot of dirt but I also wondered if it's the remains of some asbestos bitumen (which I presume could be still underneath the remainder of the tiles...)