Hi, Andy. Yes, it just happens that Martti has recently sent through some information on this, as well!
Stories have been told about this feature for many years. Because the original hole was a couple of feet lower than the surrounding rock and covered in lichen it was thought to be a lair. The hole was known to go deeper than just a few feet, and it was said that a rock dropped into it could be heard falling for many minutes. Children were warned not to go near it as the devil might come out and pull them into his chamber, hence the name "The Devil''s Nest" came into common use.
The Devil''s Nest was first excavated in 1981 by the municipality of Jalasjäven authorities and the Finnish Geological Institute. They took out 3 metres of rock, then hit a water-impermeable clay layer and discontinued the investigation in 1982.
In 1997, more money was found to continue the excavation. The clay layer was found to be a deposit from a glacial moraine, and once this was removed the hole continued to be excavated to its current depth of over 20 metres. Strangely, the material in the very centre of the hole is much lighter than that which surrounds it, and this has been christened the ''heart'' of The Devil''s Nest.
Martti has provided a link to read more about The Devil''s Nest excavation with a diagram which clearly illustrates the layers of soil, clay and rock within this enigmatic structure (click on the link, then on the ''translate this page'' box which should come up on the screen). I should add in that in the research I've been doing, I haven't found anything (yet) about any archaeological finds.
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