Just a plume volcano pushing deep basaltic magma from deep below the crust on such amounts as to cover an area exponentially larger than Europe with a kilometer thick lava flow and accompanied by very explosive rhyolitic eruptions. It put out one and a half billion tons of SO2 yearly (not to mention CO2). It was estimated to be a crack in the earth, at times, one thousand miles in length. But yeah, just a volcano.
And then there is Chatterjee’s work on the Shiva “crater”, a structure off the coast of Mumbai about 4 times the area of the Yucatan crater and of the same relative age. Of course like all big ideas his is controversial. See this link,
http://gleeaikin.blogspot.com/2014/01/pres-obama-save-middle-class-sen-mccain.html
and Google Shiva Crater for other links.
Sorry about the wrong link. My Control C wasn’t working right and didn’t hold the Shiva URL. Here is the correct one and some others:
http://en.wikipedia.org/wiki/Shiva_crater
http://news.discovery.com/animals/shiva-impact-crater-dinosaurs.htm
And many images of craters there and elsewhere:
https://www.google.com/search?q=shiva+crater&tbm=isch&tbo=u&source=univ&sa=X&ei=i4N9U-yBNuLgsATo6YDgDQ&sqi=2&ved=0CDIQsAQ&biw=1024&bih=653