OK, I'm not very good at explaining but think about this...
sqrt(2) ^ 2 = 2
so let a = sqrt(2), then a ^ 2 = 2, working back from infinity starting with the very end (i.e. the one 'at' infinity) a = 2, you'd get y = 2.
now the end a = sqrt(2) < 2, so surely y = a^a^a^a^ etc. would be less...