A black submarine in the ocean
Tech & Auto
This Is What Really Happens When A Submarine Implodes
An implosion involves a large force crushing a vessel incapable of withstanding that force as result of a significant pressure difference between the inside and the outside.
At sea level, people experience one atmosphere (14.7 PSI) of pressure. Unlike planes and shuttles, which are meant to keep that pressure contained, submarines do the opposite.
A submarine fends off external pressures, so whatever is within can stay right around 14.7 PSI. When it can no longer handle the pressure around it, it implodes.
When a sub dives beyond the depth it is rated for, it can cause hull failure and implosion. This is essentially what happens when a sub dives past its "collapse depth."
Catastrophic failures could also occur without a sub hitting its collapse depth. Any damage or flaws in the sub's construction may create a weak point and cause the hull's failure.
At extreme depths, an implosion occurs faster than your brain can comprehend what has happened. Because of the forces involved, any failure would be instantaneous.