In cave diving, a Torricellian chamber is a cave chamber with an airspace above the water at less than atmospheric pressure. This is formed when the water level drops and there is no way for more air to get into the chamber.
A Torricellian chamber in cave diving is a space within a cave where there is an air pocket above the water but at a lower pressure than normal, due to the water level dropping and no way for more air to enter the chamber.