It is still clear that the bulk of the damage happens quickly, and that nature then begins to recuperate. After a few years, a casual observer visiting a hard-hit location might see nothing amiss. Birds and fish are likely to have rebounded, and the oil will seem to be gone.

But often, as Dr. Short and his team found in Alaska, some of it has merely gone underground, hiding in pockets where it can still do low-level damage to wildlife over many years. And the human response to a spill can mitigate  or intensify  its long-term effects. Oddly enough, some of the worst damage to occur from spills in recent decades has come from people trying too hard to clean them up.

It is hard for scientists to offer predictions about the present spill, for two reasons.

The ecology of the Gulf of Mexico is specially adapted to break down oil, more so than any other body of water in the world  though how rapidly and completely it can break down an amount this size is essentially unknown.

And because this spill is emerging a mile under the surface and many of the toxic components of the oil are dissolving into deep water and spreading far and wide, scientists simply do not know what the effects in the deep ocean are likely to be.

Still, many aspects of the spill resemble spills past, especially at the shoreline, and that gives researchers some confidence in predicting how events will unfold.

Remarkable Persistence

In 1969, a barge hit the rocks off the coast of West Falmouth, Mass., spilling 189,000 gallons of fuel oil into Buzzards Bay. Today, the fiddler crabs at nearby Wild Harbor still act drunk, moving erratically and reacting slowly to predators.

The odd behavior is consistent with a growing body of research showing how oil spills of many types have remarkably persistent effects, often at levels low enough to escape routine notice.