What is the main conflict in The Woman in the Dunes by Kobo Abe?

The novel follows an isolated schoolteacher, Niki Junpei, tired of living the same routine with no purpose in his life, who leaves for a vacation and never returns. He intends to find an undiscovered beetle by the sea and earn himself a place in encyclopedias by its discovery and thus justify the significance of his life. However, after he passes through a small, poor village and sees a number of deep holes with houses at the bottom, an old man comes up to him and asks him what he is doing.