Final answer:
A downward-growing speleothem is known as a stalactite, which forms from the deposition of calcium carbonate as water containing CO2 drips from a cave ceiling and evaporates.
Step-by-step explanation:
A downward-growing speleothem is known as a stalactite. Stalactites are formed from the precipitation of calcium carbonate (CaCO3) when water that has seeped through limestone deposits and absorbed atmospheric CO2 drips from the ceiling of a cave. The process involves the dissolution of limestone by the acidic solution containing CO2, followed by the evaporation of water which leads to the re-depositing of calcium carbonate as the concentrations of CO2 in the air of the cave decrease. Over time, this results in the formation of these hanging formations that grow downward from the roof of the cave.