A search is under way for British couple Paul and Rachel Chandler amid fears their yacht, the Lynn Rival, has been captured by Somali pirates.
The case has put piracy off the Horn of Africa back in the headlines and comes 11 months after the seizure of the supertanker Sirius Star drew the world's attention to the issue.
Since then, global navies have been rushing to protect one of the world's most important shipping lanes.
Major powers have been also been debating a long-term solution to the hijackings in what have become the most dangerous waters in the world - accounting for more than half of all incidents so far this year.
How do the pirates seize the ships?
The pirates are very good at what they do.
They run sophisticated operations using the latest hi-tech equipment such as satellite phones and GPS.
So far in 2009:
147 incidents, compared with 63 in the same period in 2008
32 vessels successfully hijacked
533 crew taken hostage
85 vessels fired upon
Source: International Maritime Bureau, October 2009
They are also heavily armed with rocket-propelled grenades and AK-47s. The pirates are known to receive tip-offs from contacts at ports in the Gulf of Aden.
They use speedboats with very powerful outboard motors to approach their target. Sometimes the speedboats are launched from much larger "mother ships" on the high seas.
To actually hijack the ships, the pirates first use grappling hooks and irons - some of which are even rocket-propelled - and climb aboard using ropes and ladders. The pirates have also on occasion fired at the ships to scare them into stopping, so it is easier for them to board the vessel.
The pirates then sail the hijacked ship to the Somali pirate hub town, Eyl. There, pirates usually take the hostages ashore where they are normally well-looked after until a ransom is paid.
Why can't the pirates be stopped?
Warships from at least nine countries are now operating in the Gulf of Aden and the waters off Somalia, but this may have only shifted the problem.
The Sirius Star was attacked a long way south of Somalia. The targeted area now encompasses over a quarter of the Indian Ocean and so is impossible to police. The International Maritime Bureau (IMB) is advising ship-owners to adopt measure such as having look-outs or travelling at speeds which would allow them to outrun the pirates.
However, the pirates move extremely quickly and often at night and so it is often too late before the crew has realised what has happened.
Once the pirates have taken control of a ship, military intervention is complicated because of the hostages on board.
There is also no international legal system for people accused of piracy, although some have been put on trial in Kenya, while one group was captured by French forces and taken to face justice in France.
Some argue an international court is needed, backed by the UN, with perhaps even an international prison for those convicted.
In 2008, French commandos detained Somali 'pirates' soon after the release of 30 hostages
In mid-December 2008, the UN Security Council approved a resolution allowing countries to pursue Somali pirates on land as well as at sea - an extension of the powers countries already have to enter Somali waters to chase pirates.
But as long as Somalia continues to exist without an effective government, many believe lawlessness within the country and off its lengthy coast will only grow.
And as if to prove that, just days before the Lynn Rival went missing, a Chinese cargo ship, the De Xin Hai, was hijacked with 25 crew on board.
How big is the problem?
The IMB says there has been an "unprecedented increase" in Somali pirate activity in the first nine months of 2009.
So far, there have been 147 incidents in the waters off the Somali coast and in the Gulf of Aden, compared with 63 for the same period last year. A total of 533 crew members have been taken hostage.
The IMB also says the pirates appear to have "extended their reach, threatening not only the Gulf of Aden and east coast of Somalia, but also the southern region of the Red Sea, the Bab el Mandab Straits and the East Coast of Oman".
But Capt Pottengal Mukundan, director of the IMB, said that despite this apparent spread, the naval vessels operating off the Somali coast were playing "a critical role in containing the piracy threat".
And he said better security measures by vessels had also made it difficult for pirates to succeed in their attacks.
Why do the pirates do it?
For the money.
Chinese cargo ship, the De Xin Hai, was seized earlier this month
The pirates treat the ship, its cargo and its crew as hostages and hold them for ransom.
The rewards they receive are rich in a country where there are no jobs and almost half the population needs food aid after 17 years of non-stop conflict.
The Kenyan foreign minister estimates that pirates have received $150m in the past year in ransom payments.
They use some of this money to fund future operations - more powerful weapons, bigger, faster boats and more sophisticated equipment.
Some of the pirates are former fishermen, who say they have been put out of business by trawlers from around the world taking advantage of the lack of government in Somalia to scoop up all the fish in its territorial waters.
How does piracy affect people outside Somalia?
Unless they are involved in the shipping industry, the main effect is higher prices.
Shipping companies pass on the increased costs - security, higher insurance premiums, ransoms and extra fuel for longer routes - in their fees and so it eventually finds its way onto the high street.
Piracy is estimated to have cost the world an estimated $60 - 70m in 2008.
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