Mirror Pool

The Mirror Pool is a magical lake hidden in a cavern underneath the Everfree Forest. It appears in the episode Too Many Pinkie Pies, and anyone who looks into their own reflection while chanting the following will be cloned:

"Into her own reflection she stared, yearning for one who's reflection she shared, and solomnly sweared not to be scared at the prospect of being doubly mared."

However, the clones do not appear to be perfect duplicates; Pinkie Pie utilized the Mirror Pool in an attempt to spend time with each of her friends, and although Pinkie's clones had her personality, they did not have any of her memories as they did not know where anything was or who any of Pinkie Pie's friends were. After Twilight Sparkle used her magic to send all the Pinkie Pie clones back into the Mirror Pool, she and Big McIntosh blocked the entrance with a large boulder.