For centuries, the Bohai Sea functioned as the front door to northern China. During the Tang Dynasty, it linked the imperial court to the Bohai Kingdom, a medieval state founded by the Mohe people in what is now northeast China. Trade and diplomatic missions between China, Korea, and Japan crossed these waters regularly. Under the Ming Dynasty, when Japanese pirates known as wokou terrorized the coast, the Bohai region became a fortified frontier. Ports like Yingkou and the Dagu Forts grew from fishing villages into military strongholds guarding the approaches to Beijing. By the Qing era, the sea sustained a thriving domestic trade in salt, grain, and fish that fed the capital and its hinterland.

The last decade of the 19th century turned the Bohai Sea into a theater of war. In 1894, during the First Sino-Japanese War, the Qing navy's Beiyang Fleet suffered a devastating defeat near the Yalu River approaches. Japanese forces then pushed into the Bohai region and captured the fortified naval base at Weihaiwei on the Shandong Peninsula in early 1895, collapsing Qing maritime defense and forcing the humiliating Treaty of Shimonoseki. A few years later, during the Boxer Rebellion, an international alliance of eight powers stormed the Dagu Forts at the western edge of the Bohai Sea in June 1900, opening a path through Tianjin to occupied Beijing. These waters, so placid from above, have absorbed the shockwaves of China's most painful modern encounters with foreign power.

The Bohai Sea divides neatly into three major bays: Laizhou Bay to the south, Bohai Bay to the west, and Liaodong Bay reaching north toward Manchuria. The Yellow River empties into Laizhou Bay, carrying sediment that has been reshaping the coastline for millennia. The Bohai Strait, bounded by the Changshan Archipelago between the Liaodong Peninsula and the Cape of Penglai on Shandong, is subdivided into several channels. The widest and deepest, the Laotieshan Channel, handles the heaviest traffic. The shallowest, the Dengzhou Channel near shore, skirts the edge of ancient navigation routes that Tang-era sailors once followed.

Beneath the shallow seabed lie some of China's most productive offshore oil reserves. The Shengli Field, exploited since the 1960s, still produces roughly half a million barrels per day, though output is declining. The China National Offshore Oil Corporation was largely created to manage production in this region. In 2024, CNOOC announced the discovery of a new 100-million-ton oilfield in the Bohai Sea. Environmental costs accompany the wealth: three separate oil spills occurred within a two-month period in 2011 alone, a reminder that the sea's shallow waters concentrate pollutants as effectively as they once concentrated trade.

In 1992, officials in the port city of Yantai first proposed a tunnel beneath the Bohai Strait to connect the Shandong and Liaodong peninsulas. The idea resurfaced in 2011 with an announced plan for a 106-kilometer road and rail tunnel -- twice the length of the English Channel Tunnel. By 2013, the proposed route had stretched to 123 kilometers between Dalian and Yantai, with a price tag that ballooned to 300 billion yuan, roughly $43 billion. As of 2019, construction had still not begun. The tunnel remains one of those grand infrastructure visions that says as much about ambition as about engineering, linking two peninsulas that geology separated 27 million years ago.