The Open Guide to London: the free London guide

Quoted from http://www.islington.gov.uk/whatson/whatson.asp?sectionid=745:

Mentioned by Fitzstephen as early as 1174, the Clerk's Well was the scene of medieval miracle plays performed by the Parish Clerks of London, and gave its name to the district of Clerkenwell. Until the reformation, the well was located in the boundary wall of St Mary's Nunnery. After the dissolution of the nunnery and the destruction of its boundary wall, the well was located in the basement of a building in Ray Street (now Farringdon Lane). In 1800 a pump was placed at pavement level to facilitate public use but by the middle of the 19th century the well had been closed.

The exact location of this important site was uncertain but was rediscovered in 1924, during building work in Farringdon Lane. After renovation in 1984, the Clerk's Well now has an accompanying exhibition outlining the history of the well and its environment.