Rumours of underground tunnels always count as folklore don’t they. (It reminded me of that at Fiddler’s Hill in Norfolk).
Arbury Hill is an ancient encampment. The great earthwork was thrown up by the Romans and the whole was surrounded by a wide ditch 20 feet deep enclosing about 10 acres. There are subterranean passages which were discovered a few years ago when a dog disappeared after a rabbit, in, it was thought, a rabbit hole. The dog was called, but never came back, though it was heard barking under the ground a long distance away. Digging operations revealed an underground passage with foul air in it. The dog was never recovered.
From the ‘Northampton Mercury’, 20th October 1933.
Also (I have not read it) John Walbridge’s article in ‘Mercian Mysteries’ (1991) suggests Arbury Hill is the ‘omphalos of England’ being further from the sea than anywhere else in the country. Which could be a fun fact.