Very interesting, just did some googling on it. When you first...

Very interesting, just did some googling on it. When you first said it I was like "my god we're all going to die!"
But now is see it's not what I thought... basically the shifting of large weights to different locations on Earth (different points of latitude and longitude) can affect the Earth's rotation and wobble... So therefore we can only assume that large mining activities, damns, and even rainforest clearing via sheer movement of biomass could all have similar effects. Although the amount of time it affects would vary largely depending on where on Earth it occurred, what it was, and how much it weighed.

Very cool.