Llyn y Fan Fawr (Welsh: 'great lake (near) the peak')[1] is a natural lake in the county of Powys, Wales. It lies at the foot of Fan Brycheiniog,[2] the highest peak of the Black Mountain (WelshY Mynydd Du) range within the Brecon Beacons National Park. Created as a result of glacial action,[3] it is one of the largest glacial lakes in southern Wales.[4] The lake is roughly rectangular in shape with its long axis oriented roughly north-south.[citation needed]

| Llyn y Fan Fawr | |
|---|---|
| Location | Powys, Wales |
| Coordinates | 51°52′52″N 3°42′0″W |
| Type | natural |
| Primary outflows | Nant y Llyn, tributary of River Tawe |
| Basin countries | United Kingdom |
| Surface elevation | 605 m (1815 feet) |
The surface of Llyn y Fan Fawr lies at about 1815 feet or 605m above sea level.[5][6] Its primary outflow is to the River Tawe.[7] Much of the land around the lake is peaty and thus wet underfoot.