Vyrnwy estate around Lake Vyrnwy up for sale
A 23,00-acre (93 sq km) site featuring conservation areas and a nature reserve is being sold by a utility firm.
It is hoped the Vyrnwy estate, a popular tourist spot, around Lake Vyrnwy in Powys will fetch £11m.
The Severn Trent land, which includes several sites of special scientific interest, is being split into four lots by property consultant Knight Frank.
The consultant said it was the largest section of land to be sold for many years.
The Liverpool Corporation built Lake Vyrnwy, a reservoir, in the late 19th Century to supply water to the city.
The old village of Llanwddyn disappeared under the water but ruins can still be seen when the level drops during a dry summer.
Clive Hopkins, Knight Frank's head of farm and estate sales, said: "Never before has a property of this size come to the open market in one block under a single ownership, not in my lifetime anyway."
He said interest was anticipated from a wide variety of institutions, landowners and investors due to the land being "such a diverse portfolio of property".
The largest parcel of land for sale includes 12,000 acres farmed jointly with the Royal Society for the Protection of Birds (RSPB).
Other sections include 14 farms, a 5,000-acre area of woodland managed by Forestry Commission Wales, and 31 residential and commercial properties.
The land will be available on a 125-year lease, with water rights for the lake itself subject to separate negotiation.
The water is used to serve United Utilities customers on Merseyside.
Severn Trent chief executive Tony Wray said: "Other organisations exist that can concentrate more intensively on running the land at Vyrnwy.
"A responsible transfer process will bring benefits for all those living, working on and visiting the estate."