A submerged crannog, an artificial island built from timber, stone, and compacted organic material, typically constructed as a defensive or high-status dwelling during the early medieval period, this one at Corglass is almost invisible, lying just below the waterline rather than rising conspicuously from the shallows.

What the site lacks in visibility it compensates for in quiet strangeness. The crannog measures roughly 21 metres north to south and just over 16 metres east to west, and its low cairn of stones rises only 1.4 metres above the soft lake-bed beneath it. Rather than a firmly built-up island, what survives appears to be a scatter of stones settled on and around a small number of horizontal timbers, the whole structure having subsided or perhaps never been especially prominent above water to begin with. More striking still is what surrounds it: a large number of fossil timbers, ancient wood preserved in the anaerobic conditions of the lakebed, radiating outward from the main structure. These timbers hint at construction activity or the collapse of earlier wooden elements, and their preservation in the lake mud may hold considerable dating potential, since waterlogged wood can survive for thousands of years in such conditions.