From its shape this is almost certainly a dene hole Theye are not all that uncommon in areas with chalk near the surface. Some are quite ancient, but they were dug well into the nineteenth century and possibly even the twentieth century, to extract soft, pure chalk for spreading on the land. The bell shape is a natural enough form, since you don't want to dig a big hole until you get down to the layer you want. Traditions about dene holes are numerous (e.g. they were dug by primitive man - or as hidey holes from invasions by the Danes - see Wikipedia), but rumour and invention always gather around things of unrecorded origin. Unrecorded because everyone in agriculture at the time knew what they were and no one else ever took an interest in what the horny-handed were doing.