# Where do the underwater bubbles come from when you shoot a bullet into water, or from a spinning propeller?

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In Saving Private Ryan there are scenes where the soldiers are underwater and bullets are shown going into the water with bubbles trailing behind them.

In films with submarines, the propellers often have bubbles coming out.

Where are these bubbles, or the air inside them, coming from?

In: 5

This is known as cavitation. As a bullet travels through water, or a blade spins quickly through water, they create pockets of low pressure behind them. If the pressure is low enough then the water turns from liquid to gas, and this is the source of the bubbles. The bubbles are composed of water vapor.

If an object enters the water, some air is dragged behind it ad the object breaks the surface of the water. There’s also a thing called cavitation that affects underwater propellers (and probably bullets as well): a fast moving object can “rip” the water apart, creating a bubble (cavity, hence the name) filled not with air but with vapor from surrounding water. These bubbles collapse quickly but they are still visible.

However, don’t look into Hollywood scenes too much, most of the bubbles are just added for visuals. Actual propellers are made to reduce cavitation since it can damage the blades. Maybe some submarines have some sort of exhaust near the propeller but I won’t make any guesses here.

The bubbles contain steam or water vapor. It is caused by *cavitation* which is when the local pressure in the water is so low that the water basically boils. With a propeller you are creating high and low pressure areas behind and in front of it. This pressure difference provides the thrust which *propels* the boat. If the low pressure falls below the vapor pressure for the water temperature, it boils and forms a bubble. This bubble will sustain until it moves into a higher pressure area where it will collapse. If it collapses near the propeller, the shock-wave can actually melt a small bit of metal and over time this can result in holes.

With bullets there is a low pressure area right behind it which causes similar bubbles.

For one, films are not necessarily accurate. With a bullet being shot into the water though I do have an explanation. A fast moving object pushes air out of the way and leaves a low pressure zone behind it. Air wants to move to these low pressure zones. When it is shot into water you can actually see this low pressure zone of air behind the bullet before it fills back up with water.

With a propeller that is already submerged I only have an educated guess. There are air bubbles in water. The propeller pushes the water out of the way and since air moves quicker than water, the bubbles of air fill that low pressure zone before water can.