Hidden East Anglia:
Landscape Legends of Eastern England
Actually, this one's not so secret - plans of the tunnel exist in the Cambridgeshire Archives, it's a known feature of a Listed building, and the parish council in 2007 were concerned that it might collapse under heavy traffic.
Cut through the chalk, it runs in an L-shape from the north side of the Old Manor House (TL420458), beneath the High Street, to the 18th century Rector's Cottage diagonally opposite. Allegedly there is a blocked side tunnel branching off towards the Swan House Inn a little further away, but there seems to be no evidence for this. In 1922 it was used to run electricity cables under the road to the Manor House.
Although some have said it must have been used for escape during the days of anti-Catholic persecution, a 19th century theory was that it was used by a Royalist vicar, who was rector from 1610 to 1644. The Puritan villagers had him removed from office for his anti-Roundhead speeches and activities, but he continued to live in his own farmstead, where the Rector's Cottage now stands.
The fact that the tunnel's height varies from 1.75 metres down to about 83cms suggests that it really wouldn't have been suitable either for escape, or for regular use. Basically, no one really knows why it was created.
Dennis Ellis Hitch : 'A Mere Village' (Hitch Publications, 1993), p.167-8.
The Brook runs through part of the village, and along the eastern side of the parish. By the 1930s its flow was sluggish, and often dry in the southern part. But when a strong flow began in early 1939, then war was declared later that year, an old story that The Brook only flowed when some great disaster was about to strike the country gained wider credence. Older people suddenly started remembering that such a flow had happened just before World War One, and the Boer War, and various other national catastrophes.
Source: Dennis Ellis Hitch : 'A Mere Village' (Hitch Publications, 1993), p.168-9.