Umihotaru also connects to the world's longest underwater tunnel which connects Kawasaki and Kisarazu.
Check it out on Google Maps.
Implications - The bridge to get to this floating island is 4.4km, once they make it across, visitors can stop and marvel at the incredible lot unlike any other in the world. If they're just looking to drive by, or rather under, the 9.5km underwater tunnel is just as fascinating. You probably would never have guessed that parking could be so interesting.