A record-breaking tunnel is being constructed under the Baltic Sea, connecting Denmark and Germany. The Fehmarnbelt tunnel, spanning 18km, will be the world's longest pre-fabricated road and rail tunnel, significantly reducing travel times between the two countries. The project, financed mainly by Denmark, aims to enhance travel links across Europe while minimizing environmental impact. Despite opposition from environmental groups, construction is set to be completed by 2029, with expectations of boosting local economies and tourism.