There is a shutoff valve at the bottom of the sea, another valve on the drilling rig at the sea surface, and a pipe connecting them. The way you close off a we

There is a shutoff valve at the bottom of the sea, another valve on the drilling rig at the sea surface, and a pipe connecting them. The way you close off a well is by a) closing either of the valves or b) pumping a few tons of sand and cement down the pipe to block the drilling hole. Here the top valve failed (with the first explosion) and the bottom valve doesn't work. The pipe broke when the platform sank, and without a connection to the drilling hole the well can't be blocked in the normal fashion. Plus it's too deep underwater for divers or normal submarines. Without a connection to the well or a working valve, you're basically screwed. The only certain way to stop the leak at this stage is to drill a new hole at an angle to the first one, a relief well, then you have a connection you can use to pump in the sand and cement to block the well beneath the sea bottom. They have started the process of doing that, the only problem is that it will take months. Fixes that are known to work in shallower water, like the dome to contain the spill so it could be pumped up, didn't work here - it got clogged up by deep water crystals. Drilling at these depths is still largely experimental, so there's not much prior experience to draw on. Maybe they will find a solution at some point, but besides drilling the relief well no fix is know at this time - it's basically trial and error.