One of the most distinguishing characteristics of submarines is surprise attack. Nevertheless, H. L. Hunley went down in history as the first submarine to sink a warship, although it was not completely submerged. 

Each country pursuing naval supremacy may choose to enlarge their ships only to order them to go astern in the face of a submarine coming out of nowhere. 
What makes them that much frightening is their capability of rapid attack. Today, measures against this uninvited guest include active and passive sonars, electromagnetic anomaly detection and satellite surveillance. However, submarines employ countermeasures like prolonged stay under the sea, and less acoustic and electronic signature. 
Now, if you will, let us go back to the history of H. L. Hunley and of submarines, which launched a new era on February 17, 1864.