Thousands of caverns have been leached out from salt formations. They are used for storing a variety of fluid products ranging from compressed air and hydrogen to LPG, natural gas and crude oil, which requires that the caverns be tight. The main factors in the onset of well leakage and its prevention are discussed: fluid pressure distribution, geological environment, cementing workmanship and well architecture. The Mont Belvieu accident is described to illustrate the importance of periodic cavern testing. Test methods are discussed; apparent, corrected and actual leaks are distinguished. Factors contributing to apparent leaks are described, as are two actual in situ tests that use fuel oil and nitrogen as test fluids. It is proven that a thorough test analysis allows good estimations of actual leaks.