Another hole

Walking along the beach with the grandchildren they were jumping over The breakwaters first one hopped over second then poor Teddy jumped up and came to me crying he had hurt his leg. He said the sand went. Kissed it better. Then I went to get over and as my weight went onto the sand it gave way up to my knee. And I now understood what Teddy meant. There seemed to be a hollow under the top layer.
If we were going the other way it could have been more drastic ,as there would have been a drop the other side.
I have never seen that before in all the 17 years we have lived here.