Odd buildings in Tokyo: a Samurai helmet?

I love watching Tokyo from above, searching for known places. And sometimes, I get to discover some unusual constructions.
I noticed this odd looking structure on a roof from Roppongi... Seen from above, it kind of looks like a giant Samurai helmet (kabuto)...

Of course, I wanted to know what it is... it turns out to be a roller coaster (actually a half pipe), built on the roof of the Don Quijote Shopping Mall.
Finished in 2006, the roller coaster was never used because the residents and business owners from the neighborhood were very disturbed about having nearby all the agitation, the noise and the crowds associated with a roller coaster.
As of now, its fate is uncertain. But it sure looks interesting...