The territorial waters of the Shatt al-Arab waterway between Iraq and Iran have been central to the dispute between Tehran and London over 15 Royal Navy personnel seized by Iran's Revolutionary Guard.
Tehran claims it arrested the 15 because they had strayed into Iranian waters. The UK insists they were in Iraqi waters.
Iranian President Mahmoud Ahmadinejad has said the 15 Britons captured would be freed as a "gift" to Britain.
The Shatt al-Arab median line, which divided that waterway in half, was first agreed between the two countries in the 1970s, says Iraq expert Toby Dodge, of the University of London's Queen Mary college.
Until recent years the median line was marked simply with buoys, he says.
"But the demarcation of the waterway has suffered, like many things."
Martin Pratt, director of research at the Durham University's International Boundaries Research Unit, said shifting coastlines caused problems as it meant the median line could not be calculated precisely.
"Maritime boundaries are generally a matter of a coastline. A coastline with well defined headland is easier.
"A shifting coastline with mud flats that appear and disappear over time mean you don't have a sound basis with which to draw a median line.
"The line could shift from month to month. It makes it much harder to see with any confidence. And that is the case in this situation."
He added: "It is always dangerous to be dogmatic about a boundary unless the line is clearly defined in a treaty that is accepted by both parties."
The position at which the UK claims the incident took place lies to the south of a boundary established by Iran and Iraq in a treaty in 1975, said Mr Pratt.
However, this agreement had been "torn up" by Saddam Hussein in the 1980s.
"While the legal status and exact alignment of that boundary today are not entirely clear, it is difficult to see how Iran could legitimately claim sovereignty over the point in question.
"They have logs and charts to tell them the co-ordinates of Iranian and Iraqi waters.
"They will use GPS machines which will give them a fixed location, and plot this against charts which will tell them where different countries' waters are."
However, he said, if either of the co-ordinates that the UK says Iran supplied for the incident are correct, then "the issue is less certain".
He said: "Both points lie beyond the terminus of the 1975 boundary and there is no agreed boundary in the territorial sea.
"Based on the low-water line marked on British charts, one of the points lies just on the Iraqi side of the median line between the two coasts, the other just on the Iranian side.
"In such a context, I don't see how either side can claim to be certain whose waters the vessel was in when the arrest was made."
Richard Schofield, an expert in international boundaries at King's College London, said the fact that "there is no formally agreed boundary" caused problems.
"It isn't clear the incident happened off the water of Shatt al-Arab. We are talking about territorial waters beyond," he said.
"Iran and Iraq have never agreed a boundary of their territorial waters. There is no legal definition of the boundary beyond the Shatt al-Arab."
Craig Murray, former UK ambassador to Uzbekistan, and head of the maritime section of the Foreign Office between 1989 and 1992, believes neither side in the dispute could claim to be right.
"The truth is nobody can say if we were in Iraqi waters or if we were in Iranian waters because at this point the boundary is extremely fuzzy," he said.
"This area is extremely contested. It is an area of great dispute."
He said the UK had "made a big mistake by producing a map that has a very definite red line and saying we were definitely in Iraqi waters".
He stressed that, equally, Iran could not say definitively that the UK crew had been in its waters.
"What has happened in this case is something of a muddle. The difficulty is that in this area the boundaries, once you get inside territorial waters, are just not defined."
Mr Pratt said such boundary disputes were common, adding that in such situations negotiations are usually the best way to agree the territorial sea boundaries.
But what if diplomacy fails?
"If both sides can't reach an agreement they might submit to adjudication by the International Court of Justice or an arbitration tribunal," he said.
But he said it was a "brave step" to allow a court to make a ruling "because it takes control of sovereign territory".
He concluded: "There is certainly a need for some kind of rules to be laid down in this area. There need to be negotiations between Iran and Iraq over the boundary."
Map taken from IribNews broadcast. Specific location in relation to coastlines is unclear from the broadcast.