I never knew there was such a thing. Guess my LDS didn’t stock them. Anyway, I used to have masks that always let in water. Some a little, some a lot. I have a narrow face, so all my masks leaked. Then I saw a child’s mask in the store for $20. I asked about it and was told it was good for diving, not just snorkling and just as good in deep as shallow. So I bought it to check it out. It is the best mask I ever owned. Practically no water gets in at all, and what does is so negligible I don’t feel the need to constantly purge like I did with my other masks.