The Icehouse is an wonderful semi-subterranean domed brick structure. 
It was built more than 200 years ago and served as a year-round cold store for a hall in the days before freezers.
We use it for dining in summer, storing wine (though the labels disintegrate because of the high humidity) and the odd game of circular table tennis.
Visiting children tend to refer to it as the scary room – though that may have something to do with the rather realistic plastic rats and the (less realistic) skeleton.