Ryusendo Cave & Dragon Blue Subterranean Underground Lakes

Category: Underground Limestone Cavern & Deepest Subterranean Lakes | Location: Kaminari, Iwaizumi, Iwaizumi Town, Shimohei-gun, Iwate Prefecture

Hidden deep within the karst limestone peaks of Iwaizumi in eastern Iwate Prefecture, Ryusendo Cave ('Dragon Fountain Cave') is an underground geological wonder revered as one of Japan's Three Great Limestone Caves and designated as a National Natural Monument. While speleologists have charted over 4,088 meters of subterranean passageways—with approximately 700 meters safe and fully accessible to public visitors along elevated boardwalks—the total cavern system is estimated to extend over five kilometers into the uncharted mountain bedrock. The defining spectacle that distinguishes Ryusendo among the world's most breathtaking show caves is its series of eight colossal subterranean lakes, filled with ultra-pure limestone-filtered water that cascades from the pristine primeval mountain forests above. Three majestic underground lakes are accessible to visitors: the First Lake (35 meters deep), the Second Lake (38 meters deep), and the staggering Third Lake, which plunges to an incredible depth of 98 meters—one of the deepest accessible subterranean pools in the world. The water possesses unprecedented optical transparency; when illuminated by submerged LED floodlights, the depths glow with an ethereal, luminous deep-sapphire radiance known globally as 'Dragon Blue', creating the sensation of gazing into a bottomless cosmic portal. Beyond its abyssal lakes, Ryusendo displays a dazzling subterranean palace of stalactites, flowstones, and towering stalagmites sculpted drop-by-drop over hundreds of millennia, highlighted by the breathtaking cavern hall known as the 'Moon Palace' (Gekkyuden). The cave also provides a critical sanctuary for five distinct species of protected wild bats, including the greater horseshoe bat. The ambient air temperature deep within the limestone caverns remains at a remarkably constant 10°C to 11°C year-round, making it a delightfully cool natural retreat in the summer and a pleasantly mild haven during subzero winter freezes.