BP's reluctance to declare the capping operation a success is understandable, especially when you look at the string of failed fixes and low-ball oil flow estimates from the early weeks of this disaster. Anybody remember "top kill?” After 3 months of this environmental horror show we're all anxious to hear the words, "we have killed the well." But we're not there yet. The experts keep telling us much can still go wrong. Even if the capping turns out to be an unqualified success, they've got to complete the relief wells in order to close it off for good – and that too is not a sure thing.

On tonight's broadcast we will look at the factors behind the "cautious optimism" being expressed by the folks in charge of the operation and the unknowns that are still keeping them awake at night.

In the meantime the rest of us -- especially those who live and make their livings along the Gulf -- can only look at the underwater video and take comfort in the fact that at least for now there is no oil escaping into the sea. Tonight we'll also hear from some of those residents, and look at the cleanup challenge ahead.

I hope you're having a good Sunday. We'll look for you later on NBC Nightly News.