I've always enjoyed unagi, which is the Japanese word for fresh water eels. Never in my life do I think I've had it fresh.

Eel sauce is that sweet and salty brown sauce that you see drizzled over all kinds of sushi rolls these days. But is it ...

So what makes these eels so popular? And why are they so expensive? People in Japan have eaten eel for thousands of years. Restaurants like this can sell 40 to 50 tons of eel each year.