Irimizu Cave.

While the nearby Abukuma Cave offers a brightly lit, easy-walkway experience, Irimizu Cave is its wilder, more adventurous sibling. Formed by water flowing from Mount Otakine, this National Natural Monument is largely preserved in its raw state, trading paved paths for a hands-on caving expedition. Divided into three courses of escalating difficulty, the 900-meter-long limestone cavern forces you to wade through an ankle-deep underground stream that hovers around 10°C. Past the entry zone, the artificial lights disappear entirely, requiring headlamps, helmets, and a change of clothes to navigate the pitch-black, slippery passages. It is a wet, physical, and thrillingly unpolished climb through narrow crevices and pristine stalactites, perfect for those who prefer genuine exploration over a sanitized tourist walk.