Llyn Geirionydd lies in a valley in North Wales where the northern edge of the Gwydyr Forest meets the lower slopes of the Carneddau mountains. The lake is almost a mile long and covers an area of 45 acres (180,000 m2), but is never any deeper than 50 ft (15 m).
The lake has a car park (with toilets) and the location is very popular in the summer. This car park site was once a waste tip site for the Pandora mine above, and indeed the planting of conifers in the area of the lake has considerably softened the effects of mining. There are few, if any, fish in the lake, and this, it is believed, is the result of the poisoning of the waters from the adjacent metal mines.
The current road follows what some believe to be part of Sarn Helen, the Roman road which ran southwards from the fort at Canovium (Caerhun, between Trefriw and Conwy) to the fort at Tomen y Mur (near Trawsfynydd), and beyond, ultimately reaching Moridunum (Carmarthen).