I have been to some underground caves in Lebanon and Austria, and these were just as fascinating! These caves were much better for walking, and believe me, we did a lot of walking! We did this along with our side trip to Taxco. It was well worth it, but please wear tennis shoes or hiking boots--your feet will thank you later!
The admission is about $3, and there are guides once you are there who, for a small tip, will guide you to see the amazing formations, stalagmites, and other formations underneath. It is really hard to take any photos, and in fact, if I remember, it isn't even permitted, although I saw some not caring!
It is about a 2.5-hour drive from Acapulco.