I never knew this but there is a whole hobby culture for designing natural looking aquariums. The aquarium “guru” for this art is Takashi Amano. For a few years now they have had a world-wide contest called the Aqua Design Amano (ADA) Aquascaping Contest.
Here are a couple photos of some of the winners from 2007:
These aquariums are quite stunning. Top ten winners here.
I had an aquarium once and it never looked like that. Not even close.