What is submarine & why do we need it ?

Imagine you are on-board a ship & what would be the worst thing that would happen to you ? – the water flooding inside the ship making it sink to the seabed…damn…! So we need something which ensures either it doesn’t sink OR even after sinking, all be safe.
So in simple words – a Submarine is a vehicle which allows us to travel on water as well as deep inside the seas & oceans.
Also a submarine is used in military for protection purposes. more “How does a submarine work ?”