Uranium (U) is a radioactive element which decays to produce thorium (Th). Unlike thorium, uranium is soluble in solutions that percolate into the soil. In this way, when stalagmites are formed, they contain uranium 238 and 234 but no thorium. After crystallisation, Th-230 forms from the uranium, increasing with its half life of 75,000 years, until it achieves equilibrium after approximately 400,000 years. Comparison of the respective activity of these two radionuclides enables us to determine the time having elapsed since the calcite was formed. Stalagmites develop from their base on the floor.