The aim of the project is to better understand the formation and behavior of large and complex karstic systems. This implies the development of new measurement and field methods, as well as new modeling techniques allowing a detailed analysis of field observations and hypothesis testing. The field site for the project will be the Yucatan peninsula in Mexico, since it is one of the largest karstic systems in the world and because a large amount of data is already available even if, as we will see in the proposal, some key information are still missing. The research involves two main scientific questions: • How such a massive karstic system has formed? • How is it functioning today? These questions are not yet answered satisfactorily. The formation of the system is thought to be mainly the result of calcite dissolution enhanced by mixing of salt and fresh-water, but some authors argue that vertical heat transport in the peninsula enhances the mixing and this thermo-haline convection could be the main driving force for the formation of the vertical structures named cenote. Other authors disagree and reject the idea that heat is an important factor in the process. In terms of the current functioning of the aquifer, its detailed dynamic is not yet well understood and modeled because it involves complex interactions between freshwater, seawater, and tidal effects under a very small head gradient. All the current models of the system could not account for all those processes and they provide therefore only a general description of the overall behavior. To go a step further, we propose in this project to focus on the two main aspects. • We first aim at understanding the evolution of the permeability and the genesis of the karst system of the Yucatan peninsula over geological time scales. It includes in particular the understanding of the long term effect of the interaction between fresh water and salt water on the dissolution as well as the role of the thermal regime. • We then aim at understanding the properties of the regional-scale preferential flow paths. More precisely it involves understanding the geometry of the current conduit systems and how it influences the regional flow and solute transport. The geometry is a result of the speleogenesis processes and therefore the two questions are intimately related. Trying to answer those two questions requires the acquisition of additional data to reduce the current lack of hydrological monitoring data. It also requires the development of new groundwater flow and transport models accounting for turbulent flow in the conduits and exchanges with the matrix as well as considering heat transfer and reactive transport to allow modeling the dissolution of the calcite over geological scale. This tool will allow to compare various assumptions and test whether the geothermal convection plays a significant role or not. To reach those objectives, the project involves three teams: the University of Neuchâtel which will coordinate the project, the Geological Survey of Austria which has extensive experience in geophysical exploration in the region, and the Centro del Agua in Mexico which has a strong finite element modeling expertise.