Some analysts believe BP will eventually pay between 30 billion and 60 billion for the worst oil spill in U.S. waters.

The underwater leak in the Gulf of Mexico was plugged two weeks ago, after a variety of BP's failed attempts to kill the leak, which was triggered by the April 20 explosion on BP-leased Deepwater Horizon drilling rig off the Louisiana coast.

BP aims to start the so-called "static kill" on Sunday or Monday, a method involving pumping heavy drilling fluids, known as mud, through the blowout preventer valve system that sits on top of the well, and then injecting cement to seal it.

However, the ultimate solution is drilling relief wells. The first of the two relief wells under construction is expected to be completed in August, yet it is still contingent on the weather.

Though the blown-out well is finally on the verge of being sealed, years of legal wrangles and probes lie ahead, and myriad questions remain about the long-term effects of the massive oil spill on wildlife, the environment and the life of Gulf residents.

BP is beleaguered by a variety of investigations. The U.S. Securities and Exchange Commission and the Department of Justice have launched "informal enquiries" into securities matters related to the spill.

The U.S. Senate is also pondering a probe into whether BP played a role in the release of Abdelbaset Ali Mohmet al-Megrahi, a Libyan national and the only man convicted over the 1988 Lockerbie bombing that killed 270.

Meanwhile, several U.S. government agencies are reportedly preparing a criminal investigation into whether BP and at least two other companies committed crimes in the leadup to the Gulf of Mexico incident, such as making false statements to regulators, obstructing justice, or falsifying test results.

There are of course piles of private lawsuits and a probe by Texas authorities into possible excessive pollution in April and May at a BP refinery in Texas City, which suffered a deadly explosion in 2005 that killed 15 and injured 180.

Though the well has been finally plugged and the visible effects of the oil spill have largely disappeared, scientists are still assessing the long-term environmental damage.

An argument is already brewing over the long-term impact, with some scientists warning that the whole biological network in the Gulf of Mexico could be changed by the spill and others saying the effect could be quite small.

Some scientists cautioned that a large amount of runaway oil is still in the Gulf but its quantity and whereabouts remain unknown. They feared that much of the oil had been trapped below the surface following the use of millions of liters of chemical dispersant. Evidence has been found that large clouds of oil suspend in the water.

As this oil can affect the food chain and deplete oxygen in the water, scientists say that they want to know how fast the oil is being eaten by microbes and being diluted and where it is heading for.