Llyn Tegid or Bala Lake was the largest natural body of water in Wales until its level was raised by Thomas Telford to support the flow of the Ellesmere Canal. It reaches a depth of 30m. According to local legend, there was a holy well in the middle of what is now the lake, with the old town around it.

Sources include:
Jones, F, 1992, The Holy Wells of Wales, 192

T. Driver, RCAHMW, 17th July 2008.