The origin of Pian Grande of Castelluccio, like that of the other basins, is the result of the remote and impressive collapse of limestone along fault lines represented by the current ridges which border the plain. The main karst morphologies, originated on ancient lake and marsh deposits, are represented by various cylindrical dolines, with an impermeable bottom and supplied with water for many months of the year, funnel-shaped dolines with a permeable bottom, and especially, by the Mergani Pit. The latter karst formation extends along the most southern part of Pian Grande and carves the sedimentary bottom of the plain connecting numerous funnel-shaped dolines, the last of which forms the Mergani Sinkhole, situated just under the road which descends to the plain, from where the entire karst pit is visible.

The water which feeds the Mergani Pit originates exclusively from the melting of snow and the rain, there are no sources in the entire Pian Grande. The sinkhole is locally known as Merga. Oral tradition tells of an enormous siphon inside the mountain which fills and empties every seven years causing low flow or abundant flow phenomena at the Torbidone springs, close to Norcia.