Clerkenwell get its name from a spring (one of many in the area on either side of what was the valley of the lost river Fleet). The spring formed a well which was regarded as sacred - or at least efficacious for good health. It became the site of an annual pilgrimage of clerks from the old walled city of London who would perform mystery plays there.
Now it looks like a large quite nice loo. I visited the other week - for old times sake. It is nice that it is still there - albeit in the corner of a very ordinary building
Here's a secret - if you ring Finsbury library, they have a key and will arrange access by appointment - you may even be met there by a very pleasant and learned historian.
I believe the water retains some of its old, fabled properties but drinking it is not recommended for mortals.