I'm trying to remember a series of books I read about 15 years ago, but cannot remember the author or title of any of the books. I remember the story of the first book was about a group of children who found a secret cave at the base of a cliff next to the ocean. Inside this cave was some treasure, I think it was something like the holy grail. The climax of the story involved them going into the cave to get the treasure, and nearly getting trapped inside, because of the rising tide. The first book was relatively small and simple to read, a good book for early teenagers. There were 4 or 5 books total in the series, and I remember the later books became increasingly bigger and much more complex in their material. The later books followed the same group of kids as they got older and involved them either going back in time or travelling to an alternate universe or something like that. I remember the later books being very complicated and being totally lost when I read them at that young age. Anyone have any ideas of what I'm talking about?