Enumeration of meanders. (See also meander).
Problem is to find some formula for the number of meanders or at least some good asymptotic.
As far as I understand the attention to it has been attracted by V.I. Arnold. The problem is so "everyone can understand" that there is an article by him in the math. journal for shool-children "Quant" (sorry it is in Russian. I remember it from my school years): djvu file from the site.
There are plenty papers in arXiv on the problem.
E.g. http://arxiv.org/abs/cond-mat/0003008 Exact Meander Asymptotics: a Numerical Check Philippe Di Francesco, Emmanuel Guitter (SPHT-Saclay), Jesper Lykke Jacobsen (LPTMS-Orsay)
As far as I understand from the nice book (or) by S. Lando and A. Zvonkin the problem is still open.
