If you’re lucky enough to find the Cow Rift, there’s a cow-version of the Jar of Souls right at the start which grants unbelievable exp bonuses. I got my first Cow Rift last weekend and in a three person T6 game it took me from around P201 to P205 in 60 seconds. (The rest of the 10 level Cow Rift is an exp mine as well, with a pool of yellow water right at the start of almost every floor.)
What do you suppose would happen with a level 1 character starting off in a T6 cow rift, though? Could you hit level 70 in record time? Power Leveling: Level 70… Gone in Sixty Seconds.
Now you just need to find the Cow Rift in a Greater Rift and get three friends to carry you, and great success can be yours!