I couldn’t put it better myself:
Spouting Hornis an alluring natural wonder on Kauaiâ€™s south shore. The Horn is actually a lava tube that used to flow into the sea. A hole in the top of the tube sends a spout of water rocketing into the air with each incoming wave as the water is forced into the lava tube by the incoming surf. Near by, another hole in the lava shelf blows only air, which creates an eerie moaning sound that accompanies the firing of Spouting Horn.