(Of course this is still a terrible disaster, but not quite THAT over-the-top terrible, at least...)
Here's the most frequently quoted basis for the scary theory, Gregory Ryskin from NW Univ.
But he has since clarified that he doesn't think his theories are relevant to the current situation:
And this analysis gives a mostly non-panic-inducing picture:
(Although some of the other things they discuss as the basis for this myth are less than ideal.)
According to Gary Byerly, a geologist at Louisiana State University:
"The idea that there could be a catastrophic cave in, or a methane gas explosion, that's not a reasonable worry. [...] The rock formations on top of this oil deposit have enough strength that nothing like that is going to happen."
I found similar quotes elsewhere. So... good.
That said, the high methane levels from this particular spill aren't great for the local ecosystem:
Of course, neither is the oil. But we knew that.