Welcome to the Tokyo Bay Aqua Line (or Tokyo Wan Aqua Line), which bridges the gap between Kanagawa and southern Chiba. Cutting right through the middle of Tokyo Bay, this route connects Kawasaki in Kanagawa and Kisarazu in Chiba. It cuts hours off a journey time which would otherwise involving travelling up to Tokyo and around northern Chiba.
In just 15 minutes, you can cross Tokyo Bay including 9.6km tunnel (from the Kawasaki side) which literally rises out of the seainto a 4.4km bridge that reaches Kisarazu in Chiba. But at the intersection of bridge/tunnel is the Umi-hotaru rest area which comprises shops, restaurants, amusement facilities and, above all, spectacular views of the entire Tokyo Bay area.
Since it opened in 1997, it has vastly improved access to the resort areas of southern Chiba and Minami-Boso for people living in Tokyo/Kanagawa, not to mention allow more people who live around central Chiba to commute directly into Tokyo through bus services that span the route.
The toll fee is typically ¥3,000 but cheaper when using ETC (¥2,320) and also at weekends (¥1,000 on ETC).
It's worth stopping just to take in the fact you're on an entirely artificial island in the middle of the sea. UK Top Gear fans will recall the Tokyo Bay Aqua Line featured in 2008 episode that saw Jeremy Clarkson and his Nissan GT-R 'race' Richard Hammond and James May in the bullet train, a route which went via Umihotaru and finished at Mount Nokogiri in Chiba.