My daughter loves the volcanoes - sooooo we kept going with them. We've made an ocean floor mountain range volcano. This fits with the four types of ocean floor. This one is water proof and we will be erupting it this week. Very fun to do and not too difficult but does take a few days to complete.
So what we did is take a box from BJ's (great place for boxes locally for all kinds of projects!) and then I cut one side away and cut down the other three sides. We wanted our volcano to appear in the ocean but be "active" and erupting. :) I also wanted a way for the eruption to escape the box and make it easy to clean up. First we got a large juice jug, gathered some newspaper and then we made a big batch of salt-dough. We wadded up the newspaper to widen the bottom of the bottle and then used the salt dough to mold the shape out to appear like a volcano. Once we did that and let it dry for a little bit (one day) we then made a batch of plaster of paris (a small batch is best, but it will take multiple batches. You need to work small because plaster of paris dries quickly). We had torn up strips of paper towel to dip into the plaster and then we sort of "paper mached" the outside of our salt dough and all the way down the base of the box and up the walls so it would be waterproof. We let it dry for a couple days once we got it to look how we wanted. Lastly we painted it and we added fiber fill for smoke/gas from our volcano (hot glued that on). My daughter thought it would be fun to see the smoke blow off when we exploded it (which we are planning to do by adding diet coke and mentos to the bottle). We used acrylic paints so it wouldn't wash off and sprayed the outside with modge podge. I let her take a red glitter glue bottle and squeeze it at the top of the volcano to simulate what lava would do as it ran down the side of the volcano. She really liked that activity.