Umihotaru (海ほたる), literally means "sea firefly". It is an artificial island at the bridge-tunnel crossover of Tokyo Bay Aqua-Line, which connects the city of Kawasaki in Kanagawa prefecture and the city of Kisarazu in Chiba prefecture. Tokyo Bay Aqua-Line has an overall length of 14 kilometers, which includes a 4.4 kilometer bridge and 9.6 kilometer underneath the bay, making the underwater tunnel ranked as the forth-longest in the world. Three of the world's top five underwater tunnels are in Japan.

 

 It is a glorified five-story parking lot, measuring 650 meters in length and 100 meters wide, which forms a rectangular man-made island in between the bridge and the underwater tunnel. The parking lot on the first, second and third floor on this man-made island is always full, particularly on weekends and holidays.

Shops, restaurants, cafes and amusement facilites occupies the forth and fifth floor.