Why did Henry Moore carve holes and concavities into some of his sculptures?

Henry Moore incorporated holes and concavities into his sculpture as a way to apply "nature’s principles of form and rhythm" to his work.1

During the 1930s, Moore began collecting and studying organic objects such as pebbles, rocks, shells, and bones. He sought to translate the structural qualities of these natural forms into his own artistic practice. By opening up his carvings with concavities and piercing them with holes, he moved away from pure geometric shapes toward forms that reflected the organic complexity of the natural world.1

In his later work, Moore further developed these concepts by creating symbolic correspondences between the human body and elements of the landscape, such as cliffs, caves, and hillsides. While these techniques were initially met with public shock, they became a defining characteristic of his approach to exploring the relationship between human figures and organic, natural structures.21