A recent Philippine military report suggests that two Al Qaeda linked groups have been undergoing training in scuba diving. It is believed that this training is in preparation to carry out underwater attacks on maritime assets in ports and harbours. Reportedly, Indonesia-based Jemaah Islamiyah had financed the Abu Sayyaf at least $18,500 for explosives training in 2004. The Philippine report has also noted that Abu Sayyaf member Gamal Baharan disclosed about his training with other terrorists for scuba-diver strikes. According to Baharan, Abu Sayyaf leaders Khaddafy Janjalani and Abu Sulaiman initiated the training in preparation for a Jemaah Islamiyah bombing plot on unspecified targets outside the Philippines.

The Philippines military report gains credence in the light of the fact that in 2000 the Abu Sayyaf group in the Philippines had kidnapped a diving instructor in a Sabah holiday resort and had wanted training in diving operations. Similarly, the owner of a diving school near Kuala Lumpur had reported that a number of ethnic Malays were keen to learn about diving, but being strangely unconcerned about decompression which is an essential procedure for post diving activity.

The capture of Abd al-Rahim al-Nashiri, an alleged mastermind of Al Qaeda's nautical strategy revealed that Al Qaeda could use small submersibles, underwater motor-propelled sleds that divers use and "human torpedoes" to carry out underwater attacks. Apparently as a result of confessions by Omar al-Faruq a captured Al Qaeda operative, it was revealed that he planned scuba attacks on US warships in Indonesia. This forced US officials to visit hundreds of scuba shops in Indonesia seeking information about suspicious visitors.

It is evident that terrorist groups have built up capability and expertise in undertaking underwater attacks against maritime assets. An important question arises as to where this capability was obtained. It is fair to argue that it was obtained from the Liberation Tigers of Tamil Elam (LTTE). The LTTE, fighting for a separate Tamil homeland in Sri Lanka, had developed a sophisticated strategy of attacking maritime targets like Sri Lankan navy ships and other commercial vessels. Sri Lanka has lost at least a dozen naval vessels, both in harbor and at sea, as a result of LTTE attacks. The LTTE has engaged in wolf pack tactics, using high-speed boats filled with explosives that rammed into naval vessels. Sea Tigers, the naval wing of the LTTE, emerged as one of the most ruthless and dangerous maritime groups in the world capable of using human suicide torpedo. The Maritime Intelligence Group, a Washington-based think tank, has also noted that members of the Jemaah Islamiah, had been trained in sea-borne guerrilla tactics developed by the LTTE.

Sea mines, particularly the Limpet mines are an attractive choice for carrying out attacks. Limpet mines draw their name from gastropod mollusk. Gastropods are shelled animals and its relatives include the clam as well as the octopus. Limpets cling tightly to the rocks at low tide to keep from drying out. Mines built for attaching to ships in World War II were called limpet mines because of this clinging ability.

On 10 July 1985, French secret agents bombed the Greenpeace environmental protest flagship Rainbow Warrior in Auckland Harbour. The agents had planted two limpet mines on the vessel's hull. The first blast resulted in a big hole in the engine room. The force of the explosion was so powerful that a freighter on the other side of Marsden Wharf was thrown five metres sideways. US intelligence services believe that a large number of acoustic sea-mines had disappeared from a naval base in North Korea. These could be used to attack ships in harbours and also at choke points like the Malacca Straits and Straits of Hormuz.

The underwater medium is perhaps the most difficult to continuously patrol or kept under constant surveillance. This is primarily due to the complexity of the underwater geography as also the hydrographic conditions that prevail in the medium. Therefore, underwater attacks by terrorists are of a very high probability. So far conventional weapons like guns and explosive devices have remained the weapons of choice for most terrorists groups. This is so due to their easy availability as also low cost. Besides these are relatively easy to acquire and use. But terrorist groups are now showing interest in acquiring the capability to use underwater equipment and weapons. Therefore, the threat of underwater attack to maritime infrastructure is far from rare.