How is the entrance to the Pit sealed?

The entrance to the Pit was sealed by the rising water, which formed a great lake. The narrator observed that the water had risen to within half a dozen feet of the Pit edge ^[1], and that the entrance to the vast, underground Pit, previously yawning far down under the silent waters, was now submerged ^[1]. This effectively sealed the place from which the 'Swinecreatures' had emerged, leading the narrator to believe there was nothing more to fear from them ^[1]. He also discovered a trap door in the great cellar of the house that opened directly above the Pit, confirming the connection ^[1].