For Concept 1: Mount St Helens is a good resource, however, I've always been tickled by the Maug Islands archipelago. The islands form the north, west and east edges of a caldera volcano. If you look at the Maug Islands inset on NOAA Chart 81092, you can see the bathymetry in the lagoon. You can see at one point, the volcano was still active for a little while because of the shoaler depths in the middle. Here's a video of taken of the islands: Maug Islands.

Concept 2: I can't think of any place that has geological makeup like this... Yellowstone has overlapping calderas, but they are large and not very noticeable. I'm thinking that the formation of the second caldera would destroy the first, or at least, blow out the nearest wall. I'm not sure how you would get the cross section diagram you have attached with your post considering caves are formed in sedimentary rock, not volcanic rock - unless the cave is an empty Magma Reservoir, which, I would think would be in the Earth's crust (not in the volcano itself). I did a search for geothermal caverns and the first place I came up with is Acquasanta Terme in central Italy (and even that doesn't seem to fit your description well).

For Concept 3: I would definitely look at maps of the Yellowstone Park region. The geysers, hot springs, mudpots and fumaroles are all because of the volcanism below.


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