How do i build an under ground fort?

im 14 and my little brother is turning 9 soon. we have found a place in a cliff next to a creek on my grandparents ranch. him and i want to dig a big hole about the size of bedroom but not as tall we so far have the hole dug its just not evened out yet we need ideas for a roof to make it water proof yet invisible to the naked eye thank you for your time and im free for any suggestions.