How did so many things go so wrong at the Deepwater Horizon oil-spill site? Was it human error, an act of nature, or a blend of both? And why didn't any of those great engineering ideas to stem the spill work out? There are lots more questions than answers, even on Day 50 of the disaster on the Gulf. But a couple of things are clear: First, we got into this fix because of multiple failures and miscalculations. Second, still more ills could well surface before all this is over. Here's a quick recap of what went wrong at the wellhead, and what could go wrong in the future:

1. Why did the well explode in the first place? The Deepwater Horizon oil well, 5,000 feet beneath the sea surface, was right between the exploratory drilling phase and its operational phase. Executives from BP and the other companies involved in the drilling told a Senate hearing that heavy drilling mud was removed from the well without putting on a final cement cap. That move has drawn severe criticism, because it reduced downward pressure on the well. Oil and gas blasted up the line on April 20, touching off the initial explosion. The arguments over what the companies did or didn't do in advance of the blast will play a key role in the coming avalanche of lawsuits.

2. Could this rig have been saved? Firefighters worked mightily to put out the blaze, which killed 11 workers. Some have questioned whether the tons of water and fire retardant dumped onto the rig contributed to its sinking on April 22. If the rig could somehow have been saved from sinking, that would have made the job of capping the oil leak much easier. Instead, the rig fell to the bottom of the sea, mangling the riser line that led up from the wellhead.

3. Why didn't the blowout preventer work? The five-story-high contraption known as a blowout preventer, or BOP, was supposed to be the fail-safe option to close off the leaking well. The BOP contains a series of valves that should have closed upon command, or if the oil-and-gas pressure went out of control. Oil executives voiced profound disappointment that it didn't work. So why didn't it? Technology Review points to several reports, issued years ago, that say blowout control measures that are reliable in shallow waters are not so reliable below depths of 3,000 feet or so. This particular BOP might have been damaged by debris during the rig's fall, or it might have been unable to withstand the pressure from this particular well. Even remotely operated vehicles were unable to close down the valves - which suggests that the gush has irreparably damaged the BOP's plumbing. Would a backup BOP have done any good? That's a question to be considered during the crisis postmortem.

4. Why didn't the containment box work? In early May, BP had hoped that a 40-foot-high containment box could be lowered over the well's leaking pipe and suck up the oil and gas. The problem was that the box was too big: The seawater that was trapped within reacted with the methane bubbling up from the leak, forming crystals of methane hydrate. Those crystals essentially plugged up the hose so that oil could not be sucked up ... kind of like the hair that gets stuck in a vacuum-cleaner attachment. What's more, the crystals were lighter than water, which made the box too buoyant to keep clamped over the leaking pipe. In mid-May, BP switched to a different siphoning system that brought up oil from within the broken riser line.

5. What went wrong with the siphon? The four-inch siphoning tube worked, but it could collect only a fraction of the leaking oil - 5,000 barrels a day at best. During the early phase of the oil disaster, some experts thought the total leakage amounted to 5,000 barrels a day. The siphoning operation made it obvious that much more oil than that was getting into the Gulf. To plug the leak completely, BP pinned its hopes on an operation known as "top kill," which involved pushing enough heavy drilling mud down the well to counteract the pressure of oil and gas.

6. What went wrong with the top kill? BP pushed the mud down the well for hours at a time, for three days. But the operation could never get enough mud down the hole to keep the oil and gas from gushing back up. The exercise reminded me of trying to unplug a kitchen drain by running tap water down the sink with the garbage disposal on. Gunk just came flooding back up every time they turned off the spigot.

7. Why didn't the junk shot work? One of the extra twists to the "top kill" maneuver was to throw some extra debris - say, golf balls or strips of rubber - into the drilling mud, in hopes of plugging up the blowout preventer's leaky plumbing. This is what's known as a "junk shot." Engineers told The New York Times that the junk shot didn't come close to succeeding, apparently because the debris didn't gum up the works as they hoped. BP set aside the strategy of sealing off the wellhead, and instead tried to suck up the oil using a contraption known as the lower marine riser package, the LMRP, or the "top cap."

8. Is the top cap working? Sort of. The top cap doesn't run into the methane-hydrate problems that the containment box did because it closes more tightly over the pipe leading up from the blowout preventer. Less water gets inside the chamber, which provides less opportunity for hydrate crystals to form. Methanol can also be pumped into the cap to retard hydrate formation. To attach the cap, remotely operated vehicles had to saw off the dented riser line - and that part of the operation didn't go as smoothly as hoped. After the saw got stuck, a part of the riser had to be cut off with a giant pair of shears, leaving a jagged edge on the pipe. The cap has four vents to ease the oil/gas pressure while it's being put into place and checked out. So far, only one of the vents has been closed. As a result, lots of oil is still being vented into the sea. BP says progress is being made, with 7,850 barrels of oil being collected over a 12-hour period today. However, experts say cutting off the riser line actually increased the total flow of oil, effectively making things worse ... at least temporarily. Other oil-sucking systems are being put in place, including the Q4000 arrangement that was used for the unsuccessful top kill and a free-floating riser that's designed to ride out hurricane season.

9. Who's in charge, and what have they got? Efforts to contain the oil on the surface are another story entirely, but the main concern here is whether enough resources are being brought to bear. BP is responsible for the cost of the oil cleanup, and over the past few days company executives have said they will "meet our obligations." But critics worry that the cleanup hasn't kept up with the threats posed by the spill. Today's report about an undersea plume of oil contamination stretching as far as 142 miles from the spill site raises the level of concern. Alaska marine biologist Rick Steiner, a veteran of the 1989 Exxon Valdez spill response and a longtime critic of the petroleum industry, is worried that the cleanup effort is losing steam. He complained that marine contractors have "done a terrible job" of tending the containment booms around coastal areas. Disaster fatigue could become more prevalent as the crisis continues.

10. Will the relief wells work? Experts have been saying since late April that the long-term fix for the oil leak depends on the relief wells that are being drilled beneath the seafloor. Those 18,000-foot-deep wells are supposed to intersect with the gushing well, and provide openings for BP to push mud and cement down into the leak. As of Monday, the wells have reached depths of 12,956 feet and 8,576 feet, BP said. The wells are due for completion by August, but there's no guarantee that they'll actually intersect with the original well. Some have compared the job to threading a needle, or finding a needle in a haystack. Last year, after an Australian offshore-oil blowout, it took five attempts to hit the mark. If the Gulf of Mexico situation develops in the same way, that could add weeks upon frustrating weeks to the duration of the disaster.

Bottom line? It'd be great to have some strokes of good luck for a change: a top cap that works better than expected, or a hole in one on the relief-well front. But it's most important to have the will and the wherewithal to deal with what's shaping up as a long-term disaster relief project on America's shores.

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