Complex sentence: Brothers Josef and Chrome Čeněk were interested in the karst formations in the area and so they made the opening bigger, and in January 1913, climbed down a rope into the dark underground space.
Split into simpler sentences: Brothers Josef and Chrome Čeněk were interested in the karst formations in the area. They made the opening bigger, and in January 1913, climbed down a rope into the dark underground space.