Going up through the swarm of faults and “diaclasi” (wide cracks in rocks), the mineralized water (presently located in Sentino River) meets the colder water coming from the superficial filtering and particularly from Sentino's river-bed layer.
The Big Cave Combination was created in these faults and separations of solid Limestone. It is hard to establish the exact date of its formation. However, it must to be considered that the basic element is sulphureous water and this one could ascend only when solid Limestone has been broken and cut by faults. Estimating the age of these post-orogenesis events, occurred Upper Pliocene, it is possible to affirm that the origin of earlier rooms may go back to one million 400.000 years ago. In that period Sentino's river-bed was much higher than the present position and the whole Hydrography was more than 200/300 metres high: at that altitude , along surfaces of faults into the mountain, artesian mineralised water met the colder water of Sentino , having Limestone dissolution and Chalk deposit as a result. It may be noticed that when River Sentino was at an altitude of more than 250 metres its regime was irregular and, in winter and springtime, because of the river-bed slope, floods had a great erosive force, so that water rapidly cut the rock lowering its river-bed.
Consequently the underground hydrography suffered a rapid lowering as well, originating vertical conduits and small rooms, without leaving chalk deposits. As Sentino kept going down to a lower altitude, the water of the river went less impetuously and fast; consequently the underground Hydrography got slowly lower following exactly the river's trend. The reaction zone between sulphureous water and river colder water stayed longer and longer at the same altitude allowing then the formation of that combination of the grandiose rooms we can admire today. The basic structure of Frasassi's subterranean combination is formed by 6-8 superimposed levels which represent Sentino's erosive cycles in long geologic periods.