I have experienced the same problem. The underside of the mat grew mould that would not wash off, as a result we no longer have the mat.
a few months ago
I agree with the below review. This product is not mould resistant at all and needs to be wiped dry after each use for any hope of preventing it.
about a year ago
This product developed mould very quickly (after 1.5 months of use) and repeatedly despite vigorous attempts to keep it dry and clean. Also it has ripped (after 5 months). So it hasn't lasted like I hoped it would.