These look interesting. I got a pretty mediocre pass in my Pure Maths option at uni (I studied it for three years), but I still keep an interest in the subject. The trouble is though that studying mathematical topics can be very demanding and time-consuming so I tend to limit it quite severely and perhaps look at recreational maths books and topics rather than anything too "heavy."
Here's a forum I belong to and sometimes browse;http://math.eretrandre.org/tetrationforum/index.php
Tetration is the next mathematical operation in the series which starts with addition, multiplication and exponentiation. The mathematics concerning how it behaves, though, haven't been worked out to a conclusive result as of yet so it's still an interesting topic.
What I can say though is that the numbers get big very quickly! For example, 3t3 = 3^(3^3)
=3^27 which is about 7.62559748 × 10^12.