That is not a figure of speech. When members of the Co. Kerry Field Club investigated the site in the 1940s, they found crawl passages leading off in multiple directions, with at least two chambers they could not get into at all.

The earthwork in question is a rath, the Irish term for a roughly circular enclosure, usually of early medieval date, defined by one or more earthen banks and ditches and typically associated with a farmstead. The souterrain, an underground stone-built passage or chamber complex often used for storage or refuge, sits just inside the south rampart. When the Field Club documented it in 1946, they described a first chamber measuring approximately 1.2 metres by 2.4 metres and 1.2 metres high, constructed from large stone slabs. A narrow crawl passage led from that chamber to a second room, set at right angles to the approach route. A further passage extended beyond the second chamber but was left unexplored. An additional chamber branching off the first passage was also noted and also left unexplored. The upper entrance is marked by two covering stones, the larger roughly 1.2 metres by 1 metre, with a second, smaller stone angled downward a short distance to the south, suggesting the passage changes direction as it descends. The site may be the same structure referred to in the 1840s Ordnance Survey Name Books as a cavern in Lisnacrath fort, recorded as lying a few chains north of a place called Lissnaclinna.

What remains underground at Boolacullane, beyond what the Field Club described, is still not documented. The passages that were noted but not entered in 1946 have not, as far as the available record shows, been explored since.