Hoyos negros y sus misteriosos interiores (Black holes and their mysterious interiors)

Singularity theorems constitute a major milestone of relativity. They generated a panoply of fertile lines of research with dazzling physical consequences. (Los teoremas de singularidades constituyen uno de los mayores hitos de la relatividad. Generaron una panoplia de f\'ertiles l\'ineas de investigaci\'on con consecuencias f\'isicas deslumbrantes).
Comment: 8 pages. Invited commentary on the 2020 physics Nobel prize by the Revista Espa\~nola de F\'{\i}sica (Spanish Physics Magazine). In Spanish