Tunnels* as such do not appear to be recognised on the www but exercise advice for the butt is given, such as
http://www.menshealth.com/fitness/butt-exercise, which may help (I always thought plenty of squats were essential).
Btw, why yet another new thread for this? The forum already has tunneling posted by Brent, with a great pic., on 08-23-2013
*the word I have only learned from this forum - it was always "butt cleavage"
to my knowledge