A Cave or cavern is a natural underground space large enough for a human to enter. Throughout the storyline most of the caves are empty.
Wonderful World arc
When Akira Sengoku, Shirou Mariya and Kanako Oomori first encountered a Diatryma. They flee into a small cave. Once inside they managed to stay away from the Diatryma's beak. In the end the Diatryma lost interest and left them alone.
After the Raft was destroyed by a Basilosaurus. Akira's Group stayed inside a cave for 3 days. During that time Rion Akagami's wound has healed quite a bit. Kouichi's Group came and stayed also. After a while both groups was poison by red Berries. Most of Yarai's Group died before they were healed by White Flowers. When they were fully healed, both groups headed their own way.