WASABI!

Often, when you get wasabi with your japanese food in an ordinary fast food sushi takeout place (and yes, we have one in our town), it is merely horseradish with green food colouring, alas.

But the taste of the real thing is more subtle, but it is expensive. And the following article explains why:

asahi.com has an article on how to grow wasabi.

Unlike horseradish, wasabi is not a root, but a stem.
Growing it requires running water, so they use a stone or pipe to stablize it.

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one more factoid: You don't use a steel grater, but an oroshigane

For preparing wasabi and yamaimo, graters with the surface made from shark skin were exclusively used. These have an even finer grating surface than a metal one; much closer to a sanding paper. However, nowadays non-professional cooksusually use much less expensive graters made from other metals, plastic, or ceramics.