Limestone is mainly made of calcite. When water seeps through soil it picks up carbon dioxide gas turning the water into a weak carbonic acid which dissolves limestone. The water now contains calcium and carbonate ions in solution. When this water reaches the cave ceiling, some of the carbon dioxide can escape (out gassing) . The remaining water can no longer hold all of the dissolved calcium and carbonate and some is precipitated as calcite, forming stalactites and other cave decorations (speleothems).