Fascinating and fragile environments as are the underground estuaries, need to be studied, understood and protected for present and future generations. Even if wide and abundant bibliography related to tides and their behaviour with respect to the external estuaries is available, none dealed with the estuary caves and the related hydrogeology. This paper aims to partially fill this gap presenting a preliminary study done at the Puerto Princesa Underground River (PPUR), in the Palawan Island (Philippines). The data was collected during the last expedition (November 2016) organised by La Venta, in which some of the authors took part. During the survey, the cave has been instrumented with in continuous diver data-logger devices (CTD) recording temperature (T), electrical conductivity (EC) and water level fluctuations. Longitudinal and vertical water hydrogeological profiles of the cave were realised with the aim of understanding the dynamics of the waters during different hydrogeological regimes in combination with salt water intrusion. In addition, a bathymetric profile was done to better identify the point where to realise the vertical logs. A significant rainstorm occurred during the expedition, lasting for approximately 12 hours resulting in 80 mm of rain, and its effects gave the researchers the opportunity to analyse the hydrodynamics of the flowing waters during a flood. In normal flow conditions, the longitudinal logs highlighted the presence of freshwater inlets and the vertical logs testified to a clear stratification of the waters (freshwaters at the surface and brackish waters at the bottom).
During floods, the EC and T data showed evidence of a fast substitution of the resident waters.
