Oceans cover two-thirds of the world's surface and most of this is classified by law as international waters, or high seas, where ships are free to roam unhindered, except in a few specific circumstances. Despite a raft of new anti-terrorist measures that took effect from 2002, and more that will be applied this year, the sea and the shipping industry remain attractive for terrorist operations.
So far, al-Qaeda and like-minded extremists have mainly used fuel-laden aircraft, or trucks and other vehicles packed with explosives, as their most destructive weapons. Now, one of the biggest concerns of authorities is that terrorists may strike using another vital form of transport - ships and cargo containers.
Officials and counter-terrorism experts in the United States, Europe, Asia and Australia have warned that the next step in mega-terrorism may be an attack using chemical, biological or nuclear weapons. They are also worried about radiological or 'dirty' bombs that would use conventional explosives to disperse deadly radioactive material, causing mass panic as well as significant casualties.
Those who fear such an attack believe that weapons of mass destruction and terrorism have become interlocking threats that could fuse in an extremely dangerous challenge to global security and stability, unless effective safeguards are put in place quickly. The exposure of an extensive and long-running nuclear black market that let Iran, Libya and North Korea get weapons technology from Pakistan has heightened such fears.
As a result, the laissez-faire doctrine that has applied to the high seas from time immemorial is starting to be abridged in the interests of global security. Earlier this month, the US and Liberia announced that they had agreed to new boarding and inspection arrangements on the high seas where either side has reasonable grounds to suspect that one of their ships is carrying materials related to weapons of mass destruction or items of proliferation concern.
Measured by gross tonnage, Liberia has more ships flying its flag that any other country in the world, except Panama. According to Liberian authorities, more than 2,000 vessels are on the Liberian register, which is based in the US. Under the new agreement, the US could contact the register and request the right to board a suspect Liberian-flagged ship anywhere in international waters - and do so after waiting no more than two hours for a response.