The shell is an armored steel construction with an openable top. It is located at the sea bottom where it guards the water surface above. When a surface vessel, like your patrol boat, is located above the shell, it opens its top and releases a target seeking float mine, called a jelly. The jelly will follow your ship�s movements on its way up to the surface and try to destroy you. It does not rise as fast as float mines and you can destroy the jelly by hitting it with a depth charge. The squeaky metal sound tells when the shell has been triggered.
To destroy the shell you must hit it when the top is open, or during the open/close cycle.
If the top is fully closed your depth charges will not harm the shell. The shell carries an unlimited number of jellies. You do not get any points when destroying jellies, only when the shell is destroyed.
As the shells become harder they will react faster and the open/close cycle will be shorter.
You score between 6,000 and 10,000 points when destroying a shell.