I heard from Bushkov that there was a civil war in Russia instead of the Mongol invasion. He even provides coherent arguments:
1) All the battles take place mainly in winter – the favorite time for Slavic warfare, and the worst for the Mongols.
2) An army of that size simply couldn't have existed.
3) Even if it did exist, there wouldn't have been enough provisions for the people.
4) Even if it did exist, there wouldn't have been enough provisions for the horses.
And there's more, all in all, it's very logical...
History is constantly being re-examined, but shouldn't we focus on sharing new discoveries rather than repeating what's already been studied?