[4][5][6] In late 2008, the Japan-Korea Cooperation Committee reported that the undersea tunnel could aid Northeast Asia's integration and help establish a large Asian economic sphere.

[9][10] In August 2014, South Korean and Japanese business groups supported an undersea rail tunnel to boost tourism, improve relations, and generate ₩54 trillion (US$53 billion) in economic benefits, creating 45,000 jobs.

During World War II, the Imperial Japanese government actively pursued the project to connect it with the Korean Peninsula and, ultimately, the rest of the Asian continent.

[17][18] It was argued a combined undersea tunnel and land link would help safeguard Japanese communications and shipments to and from Europe, which would be imperiled by the Pacific War.

It further refined its plans with the outbreak of hostilities with the United States and was bent on increasing its geopolitical and ethnic ties with mainland Asia through a vastly expanded rail and marine network, with special emphasis on the Korean Peninsula land bridge to connect it with its colonies.

[18] That coincided with the planning and the development of the dangan ressha ("bullet train") system by a Japanese chief rail engineer, Shima Yasujiro, who concurred on the links between Tokyo, Korea, and China.

[23] By August 1942, Japan's South Manchurian Railway Company had created plans for an 8,000 km (5,000 mi) rail network, stretching from Manchukuo to Singapore.

[18] Against that backdrop, Japan took its first concrete step for a fixed link to and through Korea to connect it with its planned vast rail network in Asia, with the construction of several bridges as well as the completion of its 3.6 km (2.2 mi) Kanmon undersea railway tunnel joining the Japanese islands of Kyushu and Honshu.

After 1943, with increasing shortages of materials, manpower, and even transportation, Japan canceled its raumordnung (spatial plan) for its vast new Asian rail infrastructure, as it turned its full-time attention to defending its home islands from invasion.

[17] In September 2002, a five-member Japanese delegation visited South Gyeongsang Governor Kim Hyuk-kyu of Korea's southeastern provincial government to discuss the proposal of an undersea tunnel.

[17][24] The same month saw comments by Alexander Losyukov, Russia's vice foreign minister for Asia-Pacific affairs, raising discussion on the tunnel project and saying that it was "something for the distant future, but feasible.

[5] In October 2009, Japanese Prime Minister Yukio Hatoyama visited Seoul and had discussions that led to two proposals, an undersea tunnel between Japan and South Korea being one of them.

Former Japanese Defence Minister and long-term Diet member Seishiro Etō was quoted after a meeting with other interested lawmakers from various parties: "This is a dream-inspiring project" and "We'd like to promote it as a symbol of peace-building.

[6] A noted longtime proponent of the tunnel project was South Korea's Sun Myung Moon, the late Korean founder who led the worldwide Unification Church.

Moon proposed a "Great Asian Highway" as early as November 1981 at the 10th International Conference of the Unity of the Sciences and helped establish several related committees over the next three years.

Since approximately 1988, three newer routes have been proposed for the project by the Japan–Korea Tunnel Research Institute Society (founded by the Korean Unification Church), with all three having the most eastern point terminating at Karatsu, Saga Prefecture, on the Japanese island of Kyūshū.

In early 2009, the joint study group stated that the route would almost certainly begin at Karatsu, in Japan's Saga Prefecture, and would likely travel to Geoje Island, on the Korean shore.

That would offer South Korea a chance to redefine and expand its tourism industry to include other cities and destinations besides Seoul, as the tunnel would serve as a gateway for tourists to travel with ease to and from the peninsula.

[37][38] The tunnel would assist in the creation of the proposed BESETO (Beijing–Seoul–Tokyo) Highway Plan, which would connect six megacities (Shanghai, Tianjin, Beijing, Seoul, Osaka, and Tokyo), each having a population of greater than 10 million.

"[34] Key issues for the tunnel would be its enormous construction cost combined with possible low profitability, similar to the Eurotunnel's financial situation since it opened in 1994.

[8] In 2014, the South Korean Busan Development Institute estimated the undersea tunnel could create 54 trillion won (원54T, or US$53 billion) in economic benefits and provide about 45,000 jobs.

Among the most significant was the loss of a key psychological barrier (akin to an "island mentality") that had held back many Britons and other Europeans from traveling to each other's nations, according to Kim and Nozawa.

Fifteen years after opening the Channel Tunnel, it was estimated that 300,000 French citizens were living in London, helped in part by reasonably priced Eurostar fares and service that is almost completely immune to bad weather and heavy seas.

"[42] The Japan-Korea Cooperation Committee, composed of business organizations and academics, similarly concluded in August 2009 that "the undersea tunnel may contribute to the integration process of Northeast Asia."

[42][45] In contrast, northeast Asia, also one of the world's fastest-growing economies, experiences a lower degree of internal political cohesion partly due to its poorer intraregional transportation links.

[47] This observation was similarly noted after a two-day meeting in late 2008 by the Japan–Korea Cooperation Committee of business leaders and academics that reported "the undersea tunnel may contribute to the integration process of Northeast Asia",[5] helping to establish an Asian economic sphere of several hundred million-plus people.

Professor Shin Jang-churl of Soongsil University in Seoul advised that it was essential for consensus to be reached by nationals of both Japan and South Korea on the relevant issues that divided them.

In the early 2000s, Japan's relationship with South Korea worsened when Japanese Prime Minister Junichiro Koizumi visited his country's Yasukuni Shrine several times, an action that was deemed offensive to many Koreans.

[48] Another contentious issue may be the territorial dispute over the Liancourt Rocks islets (Dokdo/Takeshima) to the northeast of the strait, which have long been claimed by both parties but are under South Korean control since 1952 in what the Japanese government regards as an illegal occupation.

[51] The scandal was so severe that Prime Minister Fumio Kishida reshuffled his cabinet on 10 August 2022 in an attempt to cope with a sharp decline of his approval rating after the assassination.