How could lightning have killed 322 reindeer at one time in Norway? Lightning is supposed to have killed 322 reindeer within an area that's about 50-80 meters in diameter on a mountain plateau in Norway. Nearly the entire herd died.
Sources: (1, 2, 3, 4) 
Photo: Warning: graphic content! 
Image source
I'm struggling to comprehend how this can be possible within the laws of physics. I mean, I suppose it must be, as I'm not sure if there is any other possible explanation. But I'm asking how the idea that they were killed by lightning can be explained to be possible according to the laws of physics.


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*How could the electricity travel between the animals? Wouldn't the reindeer have had to stand extremely close together? While the photo above does seem pretty crowded, it does also look like there would have been gaps between many of the animals.  Or must they all have been touching each other directly? If not, what kind of distances are possible?

*Assuming that all of the 322 animals were forming a continuous chain, would the electricity from a single strike travel all the way through all of them? Or would it instead go into the ground before going all the way through that chain?

*Are multiple strikes of lightning a possibility? How likely is this?

*Could water have been involved somehow? (Apart from the water within the animals.)
 A: The antlers of a reindeer are like head-mounted lightning conductors. Given photo indicates that the region is mostly barren, which means the highest objects in the vicinity of lightning leaders were high antlers of the reindeer. I imagine some six hundred streamers rising from the herd before the lightning strike had occurred.

Read about Lightning leaders and streamers
Streamer current has the potential to kill a human. Maybe they could kill reindeers as well. The first few deaths could have been due to this. Once the step leaders and streamers meet, lightning occurs. The ground current-induced deaths would follow. (Source)
I doubt if ground current alone could kill all the animals spread out in such a vast area. It could be that the great thunder and sudden flash of lightning strike simply took the breath out of the animals. It could have been a 'Mass heart attack'.
A: Regarding @HDE 226868 answer, it does seem like current over the surface of the ground  is the likely culprit. I recall reading that from several sources (but unfortunately I don't have a record of them).
From what I could make out of the picture of the dead animals, there didn't seem to be any evidence of a direct strike on them (no burn marks). However, it doesn't take much current to cause ventricular fibrillation and there would be no outward sign of it. My recollection is that studies using dogs showed that 60 Hz rms current of less than 100 mA is sufficient. It would seem that a direct lightning strike would easily be capable of generating currents over a wide surface area greater than that. Such currents could travel a path up the legs of the animals and through the heart.
As an aside, and this is probably just coincidence and has nothing to do with the event, my recollection back around 2002 was that Norway was one of the few countries that did not have an earthed power system. Earthing a power system keeps on the voltages on the grid from becoming excessively high if struck by lightning. 
Hope this helps.
A: The National Weather Serivce has a page on ways someone can get struck by lightning. Two possibilities jump out at me as potential killers in the case of the reindeer:
1. Ground current
Lightning might hit a tall object - a tree, telephone pole, or something else - and travel through it into the ground. After this, it may spread out in various directions through the soil in what is termed a ground current formed by a step potential; this "kills many farm animals". In particular, quadrupeds are susceptible because their feet may be spread out quite far, and so their contact points may be quite distant, meaning that the current can travel throughout their body and vital organs. See also this page.
2. Side flashes
An object that has been struck by lightning may send current to an object close to it - resulting in a secondary strike, or a side flash. If one reindeer was struck directly, I suppose the current could then jump to a second reindeer sheltering nearby. However, 321 side flashes (there being 321 other animals to reach) is quite a lot; I don't think that this phenomenon alone could kill to many reindeer.

Gomes (2012) goes into a lot more detail, including citing specific instances. The author concludes that two cases of mass sheep deaths (654 and 850, respectively) are almost certainly due to the formation of a step potential.
A: A lightning flash causes a short pulse of the magnetic field surrounding the current. This rapidly magnetic field induces a voltage in loops, for example in loops consisting of the the front legs, the body, the hind legs and the soil on which the reindeer stands, about 1 m².
The magnetic field around the lightning channel decreases as $1/r$, much slower than the current in the ground. So this mechanism can explain why so many cattle can be killed.
If $\frac{\Delta \Phi}{\Delta t}$ is of the order of 100 volts, this may cause heart stop in the reindeer.
A: A massive amount of charge has to dissipate.  Permafrost is not a good conductor so the millions of volts generate a large potential gradient over a thin, resistive, warmer surface.
Pressure melts ice, so with four points of high pressure low resistance contact each, the livestock offer a much lower resistance path for the charge to dissipate.  Perhaps the electrical energy of the strike was dissipated only down paths passing through livestock.
