The Finnish government owned Patria Advanced Solutions Oy company, has introduced a formidable new naval mine; the M2. This is a "smart naval mine, containing its own computer, and able to detect targets via acoustic (detecting the sound of the target ship), pressure (detecting the pressure on the water a ship over head makes), and magnetic (detecting the metal in a ship) sensors. Many details of this bottom (it is placed on the sea bed of shallow coastal waters) mine are classified. The Finns are manufacturing this mine for their own defense, and export to select customers (those unlikely to attack Finland). The mine is believed to have an electrical field sensor as well, and that it can be detonated by remote control. The computer in the mine is programmable, so the mine can be instructed to attack only certain types of ships.
But the most interesting aspect of this mine is that it was made with COTS (Commercial, Off-The Shelf) components. That means that many other nations, with a sufficient supply of capable engineers, could build a similar mine. It also means the M2 is probably very cheap, without the expense of a lot of custom made (and developed) parts. A weapon like this, in the hands of terrorists, would be a very dangerous weapon. As a "bottom mine," it is very hard to clear. Weighing less than half a ton, it could be transported and planted by a relatively small boat. A fishing boat would be ideal, if you were up to no good.