Cueva vs. caverna vs. gruta
In Spanish, there are three words that are often used interchangeably to describe a natural underground space large enough for a human to enter: 'cueva', 'caverna', and 'gruta'. While they all generally mean the same thing, cave, there are subtle differences mainly in their usage and context.
Cueva
'Cueva' is a generic term for any kind of hollow form or cavity in the earth. It is a broad term that can apply to any size or type of cave.
La cueva más grande del mundo está en Vietnam.
(The biggest cave in the world is in Vietnam.)
En esa cueva vivían los primeros seres humanos.
(The first human beings lived in that cave.)
Caverna
'Caverna' often refers to larger, more complex systems of caves. It has a little bit grander connotation than 'cueva'.
El sistema de cavernas de este parque nacional es impresionante.
(The system of caverns in this national park is impressive.)
Nos perdimos en la extensa caverna.
(We got lost in the extensive cavern.)
Gruta
'Gruta' often refers to smaller, often picturesque caves. They are typically associated with beauty or mythical significance.
Esa gruta tiene hermosas formaciones de estalactitas y estalagmitas.
(That grotto has beautiful formations of stalactites and stalagmites.)
"La Gruta Azul" es una famosa atracción turística en Capri.
('The Blue Grotto' is a famous tourist attraction in Capri.)
Summary
'Cueva', 'caverna', and 'gruta' are all Spanish words meaning 'cave' though they may be used in slightly different contexts. 'Cueva' is a general term and can apply to any kind of cave, 'caverna' usually refers to larger or more complex cave systems, while 'gruta' tends to describe smaller, picturesque or mythically significant caves.