The engineering classification of cross-sea bridges is based on the barrier they traverse, rather than strictly defined bridge structures.

Generally, these bridges are considered to be structures crossing oceanic traffic channels, their scale determined by the length of the channel.

In contrast to ordinary land roads or sea landing sections, cross-sea bridges take various forms.

The Öre Straits Bridge exemplifies this, connecting Copenhagen in Denmark with Malmö, Sweden. Spanning the natural division of the Öre Straits, this engineering marvel consists of an 8-kilometer bridge, a 4-kilometer road over an artificial island, and a 4-kilometer undersea tunnel.

Completed in June 2000, this bridge solidifies the eastern region of Denmark and the southern region of Sweden as the most densely populated, economically active, and culturally exchanged international metropolitan area in the Nordic and Baltic regions.

The Great Belt Bridge, a 17.5-kilometer link between the Danish islands of Zealand and Fiing, spans the Great Belt Strait.

Comprising a suspension bridge, submarine tunnel, and box girder bridge, it's often referred to as the East Bridge. Completed in August 1998 at a cost of $4.8 billion, this project was Europe's highest-budget bridge endeavor at the time.

Japan's Seto Ohashi Bridge, stretching 37.3 kilometers across the Seto Inland Sea, is an engineering marvel with three cable-stayed bridges and a truss bridge. Completed in 1988 at a cost of $8.46 billion, it was the world's longest bridge at the time. Serving both highway and railway purposes, it boasts the longest suspension bridge span of 1,100 meters.

The Chesapeake Bay Bridge, a 37-kilometer link between Maryland and Virginia, opened in 1964 and is renowned as "one of the seven wonders of the modern world."

Serving as a two-way trans-oceanic connection from Norfolk to the Delaware Peninsula, it offers stunning views of the Atlantic Bay and is accompanied by a man-made island providing recreational opportunities. Notably, the bridge opens annually for public walking and bicycling tours.

These remarkable cross-sea bridges showcase the intersection of engineering prowess and human ingenuity, connecting regions, fostering economic development, and leaving an indelible mark on the global landscape.

Crossing the boundaries of conventional engineering, cross-sea bridges stand as awe-inspiring feats of human innovation, defying the limits of structure and scale. Öre Straits Bridge, an embodiment of this engineering marvel, seamlessly links Copenhagen and Malmö, serving as a testament to the unification of Denmark and Sweden.

Its artificial island, intricate tunnels, and extensive bridge network symbolize not just physical connections but also the harmonious blending of cultures and economies.

The Great Belt Bridge, spanning the Great Belt Strait, showcases the intricate dance between suspension bridges, submarine tunnels, and box girder bridges. This colossal 17.5-kilometer structure not only binds Danish islands but also stands as a testament to European engineering ingenuity, challenging the waters that separate Zealand and Fiing.

Japan's Seto Ohashi Bridge, a masterpiece stretching over the Seto Inland Sea, is a visual spectacle with its cable-stayed bridges and truss bridge. Beyond being a transportation lifeline, it is a symbol of Japanese precision and dedication to pushing engineering boundaries.

The Chesapeake Bay Bridge, often hailed as one of the modern wonders, is more than a thoroughfare between Maryland and Virginia. Its annual opening for public tours and recreational activities transforms it into a communal space, emphasizing the bridge's role beyond connectivity to create memorable experiences.