Here's one that went Blammo:
https://wryheat.files.wordpress.com/...ngo-crater.jpg
And here's a huge one in northern Canada that hadn't exploded yet:
http://tinyurl.com/nxhposq
And one that blew out a chunk of a road:
http://tinyurl.com/ltl4p56
Silly-ish name, scary phenomenon. Some of these things get several stories high before they blow. The scariest part is that the EPA is currently repealing all kinds of "behave responsibly" regulations for the oil industry, and one of the places that's had a bullseye on it in oil-company eyes for decades is Alaska, where you bet these things are forming up as well. Only a matter of time before a rig hits one or one pops up unexpectedly under a pipeline. Which, if the info I remember isn't hopelessly outdated, would probably be in or running through Denali National Park.