Mount Calavera in Carlsbad is one of three volcanic plugs in southern Califoria. It is unique in that one side of the mountain was removed by a gravel quarry revealing the columns of dense rock that form the plug. It is now part of the Calavera Nature Preserve; a small restroom and many hiking trails were added. Some parking is available near the eastern end of Tamarack Avenue in Carlsbad.

Here is a model of the cut in the side of the mountain that reveals the innner core.