To answer your question, smaller habitat, no experiment at maintaining atmospheric composition, outside excursions in "space suits" etc. Its not very much like Biosphere II.
As for why not under the sea or Antarctica I can give at least three reasons. (1) cost of building, transporting and maintaining the habitat; (2) all the support and research personnel live in Hawaii, above water; (3) the research objectives don't require putting the experiment in a dangerous or inaccessible place.
Now someday when we have an actual habitat design along with all the actual support systems we plan to send to Mars, a trial on top of a super high mountain would make sense as a kind of Mars analog. But we don't have such stuff to test so we don't need the Mars analog with all the expense and complication.