For many decades, the famous Cacahuawamilpa Grotto was considered the world's largest caves. They are also the most-visited caves in Mexico and located in the Sierra Madre del Sur. They were discovered in the year 1834 and are chock full of stalactites and stalagmites. Visitors can only access the first 2km, because due to the type of caves, it would be very difficult for the average visitor to travel beyond that point. The environment within the caves is semi-dry and there is forest vegetation.
They've discovered "Banquet halls", 90 caves as large as 10km each! Because it is considered a national park, you can also do other activities like hiking, trekking and camping in the area.