Little is known about the Movement for the Emancipation of the Niger Delta but it has shown that it is capable of destablising Nigeria's oil industry.
Delta residents want to share in the region's oil wealth
A wave of attacks on oil installations and kidnapping of foreign oil workers has reduced ouptut by 25% and now it has expanded its theatre of operations by placing a car bomb in a miltary barracks in the regional capital, Port Harcourt.
Most of its previous operations have been in the rivers and creeks of the Delta, rather than urban areas.
Analysts say the group has shown that it has some political support from the communities of Nigeria's oil producing region, who have long complained that they were not benefitting from the riches under their feet.
It appears to be a more effective organisation than the armed groups which have been extorting money from companies working in the region and stealing oil for many years.
The explosions out in the swamps which have closed down significant parts of the oil industry were carefully placed by people who understood the geography of the pipeline network.
The BBC's Abdullahi Kaura Abubakar managed to meet one of the group's leaders, who used the alias Major-General Godswill Tamuno.
But he refused to be interviewed on tape or for his location to be disclosed.
Our correspondent says the "general" was not visibly armed and you could easily walk past him in the streets without noticing him.
Mend's leaders like to be faceless, our reporter says, and they usually send statements to the media via e-mail.
Mr Tamuno told our reporter that Mend was fighting for "total control" of the Niger Delta's oil wealth, saying local people had not gained from the riches under the ground and the region's creeks and swamps.
He said the Delta had been exploited for the benefit of other parts of Nigeria and foreign companies and ordered all oil companies and Nigerians whose roots lie elsewhere to leave the region.
This argument has been made by several other militant groups who have staged attacks in the Niger Delta in recent years.
Nigeria is one of the world's biggest oil exporters and yet most Delta residents live in poverty.
There are few major roads in the area and even fewer decent hospitals.
The group enjoys considerable local support and it is difficult to pinpoint exactly who is a member, our reporter says.
But unlike at least one other group, Mend has not specifically called for the Niger Delta to secede from Nigeria.
This was one of the demands of Mujahid Dokubo-Asari, the leader of another militant group which said it was standing up for the rights of the Niger Delta's biggest community, the Ijaws.
Last year, his threats of open warfare against foreign oil companies caused turbulence on the world markets.
He was invited to the capital, Abuja for a meeting with President Olusegun Obasanjo.
He operated quite openly and after talking about independence once too often, he was arrested and is in custody, awaiting charges of treason.
Another Mend demand has been for the release of Mr Asari but they insist they are a separate organisation.
However, Mr Asari's Niger Delta People's Volunteer Force has gone quiet recently and it is quite likely that at least some of his supporters are behind the new group.
While Mend and the other militant groups claim to be standing up for Delta residents, some locals say they are just oil thieves.
The region is home to a huge industry of stealing oil and selling it on the black market.
This trade is believed to fund the purchase of weapons.