They can actually hear each other though the land one? How long are they normally?
Well, since the tunnels interconnect with each other and with cenotes like the tunnels in a termite nest, they can be dozens of miles long.
There was a National Geographic article (10/01/2003) about divers exploring the cenotes and the interconnecting cave tunnels that we took with us to the Yucatan trip. The article had a fold-out illustration diagram that include a tunnel reaching the sea and opening at the seabed to create an "Ojo de Mar".