On highway 307 from Playa del Carmen to Tulum (Mexico) you’ll find a number of Cenotes right before you reach Akumal.
A Cenote is a natural limestone structure which contains ground water (and lots of flora and fauna).
The ride from Playa del Carmen to Cenote Cristalino is 20 Pesos by Colectivo (shared van).
Cenote Cristalino is together with Cenote Azul and Cenote Jardin de Eden. Entrance is 50 pesos per person.