I'm not positive but if I am correct, it is either your trachea and/or mucus membranes.
Grasshopper
Anther
Bean
The Acropolis in Athens.
Fort Charlotte
Copper
The Parthenon.
Yellowstone National Park sits atop a massive stratovolcano. This super-volcano is thought have last erupted around 3500 years ago. Volcanic activity remains visible and detectable at Yellowstone, but as of July 2014 there are no signs of imminent eruption.
Ceres
The building features a concrete frame which sits atop one made from steel.
high voltage sending unit that sits atop each sparkplug
It's not entirely hollow, but yes, it is a organ that sits atop the trachea