What is an open circuit scuba dive?

This is a dive using Scuba equipment you are used to seeing. It is where you breath from a scuba cylinder and exhale all the gas you breathed as bubbles. Since you are not recycling or rebreathing any of the gas, the circuit is open. Closed circuit or semi-closed circuit is where divers use "rebreathers" and rebreath the majority of their gas. 100% of the gas can never be rebreathed, so a very small amount is exhaled as bubbles.