Complex sentence: It is cold underground so there is little evaporation but some does take place leaving a trace of Limestone on the roof over thousands of years a stalagtite forms from the ceiling as the Limestone continues to drip.
Split into simpler sentences: It is cold underground so there is little evaporation but some does take place leaving a trace of limestone on the roof. Over thousands of years a stalactite forms from the ceiling as the water continues to drip.