Med ordet “grotte” menes mer enn grotter i karst. Arthur Palmer definerer i sin bok Cave geology (2007) grotte som følger:
A cave is a natural void beneath the land surface that is large enough to admit humans.
Han forklarer: Nearly all caves extend into total darkness. Many underground openings are too small to enter, and some of them have the same origin as caves, but these tiny pores and fissures are not defined as true caves because we cannot explore them ourselves.
Merk f.eks. ordene "land surface". Skal f.eks. grotter i isbreer inkluderes? Palmer svarer på det også, og mener at de teknisk sett ikke er under "the ground", men blir ansett som å være grotter.