Fern growing in a well

Fern growing in a well

Observation - Fern growing in a well. Description: Fronds about 15-20cm long. Unfortunately I didn't photograph the backs of the fronds. Some older stems appear blackish, but younger leaves have green stems. 'Coastal' not really relevant, as it was growing in a dark well (at Dunstanburgh Castle). Glos

Fronds about 15-20cm long. Unfortunately I didn't photograph the backs of the fronds. Some older stems appear blackish, but younger leaves have green stems.
'Coastal' not really relevant, as it was growing in a dark well (at Dunstanburgh Castle). Glossy, attractive leaves. Any ideas please?