While the most well-known cenotes are large open water pools measuring tens of metres in diameter , such as those at ChichÃ n ItzÃ , most cenotes are smaller sheltered sites and do not necessarily have any surface exposed water . While the best-known cenotes are large open water pools measuring tens of meters ( yards ) in diameter , such as those at ChichÃ n ItzÃ , the greatest number of cenotes are smaller sheltered sites and do not necessarily have any surface exposed water .