text: the mass extintion theory about the comet or asteroid killing the dinosaurs has fallen into question why?   i did not know of this.    i was aware of the debate surrounding the fires after the asteroid hit earth.  the link i have attached is a bbc one discussing that very same thing.  i recently watched a science programme which showed research that clearly indicated that the dinosaurs due to the initial impact, associated fire and the ash debris that covered the earth blocking plant life.    the reasons for argument had been focussed on rocks that did not show effects of fire (charcoal).  the programme i saw (which i have forgotten) showed an experiment where the angle of the asteroid at 30 degrees created an effect that they could substantiate with rock findings.  whereas initial studies surmised a direct hit with equalised spread of damage which the geologists could not support in findings.  to add to the argument found a great q&a site regarding the differing academic views - it is still under debate!  what did scientists think killed the dinosaurs before the impact theory was put forward? before the late 1970s, there were any number of different theories to explain the mass extinction, with no real consensus. one theory put it that mammals had eaten all the dinosaurs' eggs, but this has now been largely discredited. another, much more widely supported, theory was that massive volcanism caused environmental changes which drove the dinosaurs to extinction. walter and louis alvarez first proposed the impact theory in 1979. soon it would dominate all thinking on the subject.   what is iridium? does it occur naturally on the earth? iridium is an element which is found on earth, mostly in the molten mantle, the layer beneath the crust. sometimes it is ejected in volcanic eruptions, but it's still extremely rare. it is found in far higher concentrations in some kinds of asteroids. the impact theory was first put forward when scientists discovered asteroid-type concentrations of iridium in the kt boundary layers.  how do scientists explain the survival of some groups of animals during and after the impact? it depends on who you ask. some supporters of the impact theory suggest animals like frogs, turtles, crocodiles and fish survived by immersion in water or mud. this immersion, they argue, protected them from the initial heat wave and from the subsequent cold of an 'impact winter', when dust and atmospheric chemicals blocked out the sun.  mammal species also survived. the theory has it that because most of them were small they could burrow to escape the heat and the cold.   those who oppose the impact theory argue that the very survival of these kinds of animals, some very sensitive to cold and pollution, undermines the idea that there was a global environmental catastrophe at all.  how likely is that two massive asteroids could have hit the earth within 300,000 years of each other as prof keller suggests? there is evidence that impacts might come in clusters, caused by a lot of debris flying around the solar system at particular times. for example, two huge craters from about 35 million years ago were created about 200,000 years apart: popigai crater in russia and chesapeake bay crater in the usa. so although it would be unusual, it's certainly not impossible that there were two big impacts at the kt boundary.  how likely is that prof keller's team will find a crater from the second impact that they say coincided with the end of the dinosaurs?  it is possible but perhaps not likely. 70% of the earth's crust has been 'subducted' - pushed back into the mantle by another tectonic plate - over the last 65 million years. of course, supporters of the single impact theory argue it will never be found, since it doesn't exist!  why do keller's team believe that another impact caused the extinction? why don't they think it could have been something else? they believe that there were two impacts, one at chicxulub 300,000 years before the final disappearance of the din