Hydrology is among the principal disciplines within the larger framework of hydrologic sciences, itself a subcategory of earth sciences study. Of particular importance to hydrology is the hydrologic cycle by which water is circulated through various earth systems above and below ground. But the hydrologic cycle is only one example of the role water plays in the operation of earth systems. Outside its effect on living things, a central aspect of the hydrologic cycle, water is important for its physical and chemical properties. Its physical influence, exerted through such phenomena as currents and floods, can be astounding but no less amazing than its chemical properties, demonstrated in the fascinating realm of karst topography.