Tokyo Bay Aqua-Line Expressway is a stretch of road approximately 15 km long across the center of Tokyo Bay and joining Kanagawa and Chiba prefectures. The expressway opened in 1997 following 10 years of construction, which in turn had followed 20 years of surveys. When it opened, the Aqua-Line was referred to as the Apollo Project of Civil Engineering for its aggregation of what were then cutting-edge technologies. The road is comprised of the undersea tunnel (approximately 10 km) and Aqua-bridge (approximately 5 km) and an artificial island, Umihotaru, which is a parking area completely surrounded by water. The Tokyo Wan Aqua-Line Expressway Undersea Tunnel Adventure Tour offers guided tours of the undersea tunnel not normally open to the general public. Reservations required.
See website for details: https://www.umihotaru.com/ait_tanken/en/about_tours.html
神奈川県と千葉県を結ぶ東京湾の中央部を横断する約15kmの道路。調査に20年、建築に10年費やし、1997年12月に開通。当時の最新技術を結集し「土木のアポロ計画」とも言われた。海底トンネル（約10km）とアクアブリッジ（橋りょう約5km）から成り、人工島「海ほたる」は、海に囲まれたパーキングエリアだ。普段は入れない海底トンネルの裏側を探検する「東京湾アクアライン裏側探検ツアー」もある（事前予約、HP要確認）。
https://www.umihotaru.com/ait_tanken/en/about_tours.html
