Landing at Haneda I spotted this structure and surprisingly a large number of blogs and online and discussion boards believe the structure to be some secret government spy satellite.

Tokyo Bay Aqualine is the fourth-longest underwater tunnel measuring 9.6 km long from Yokohama to Chiba under the Tokyo Bay. The tunnel shaves some 100km off the round the Bay trip to Chiba and believe it or not, it took 31 years to build and cost $11 billion dollars. The Aqua Line is a 4-lane highway 

The tunnel is brightly lit and spaciously built. About half-way through, signs indicate the Kaze no To, the Tower of Wind. That’s not a point to stop, just an information that you pass below that tower which, as its name suggests, supplies fresh air to the tunnel, powered by the almost constant winds of the bay. When you reach daylight again at the end of the tunnel, you drive onto a small artificial island called Umihotaru which translates as “Sea Firefly”. The island hosts a five-story building, the Umihotaru rest stop: https://www.umihotaru.com

This is this is a ventilation shaft for the Tokyo Bay Aqualine – an undersea tunnel that lies approximately 40 meters below this structure and both are earthquake resistant construction – so they say.

Tunnel Aqualine – under Tokyo Bay 40km depth and tidy roads