Menoyu is a natural wild hot spring located within the Maruo Nature Trail in Takachikho, Makizono-cho, Kirishima City, Kagoshima Prefecture, known as the "oldest rock bath in Kirishima." It is also affectionately called the "12th Bath." As its name suggests, it has been traditionally believed by locals to be effective for eye ailments and is known as a hot spring that benefits the eyes.

The main characteristic of this hot spring is that it is a naturally occurring spring that bubbles up directly from the bottom of a bathtub carved from natural rock. It's a small rock bath that can barely accommodate 1-2 adults, with no facilities such as changing rooms, maintaining its completely natural wild hot spring state. Despite being located in a forest surrounded by Kirishima's virgin forests, one of its attractions is its excellent accessibility—just a 1-2 minute walk from the entrance of the Maruo Nature Trail.

A distinctive feature of the hot spring is that its water volume is not constant, and the color and temperature change significantly depending on the season, timing, and weather conditions. The temperature varies widely from 35°C to 60°C, making it a matter of luck whether you can bathe at a comfortable temperature. In winter, temperatures can drop to around 30°C, making it unsuitable for bathing at times. The water is mostly colorless and transparent, without the characteristic sulfur smell of sulfur springs.

Right next to Menoyu flows a stream called "Menoyu-yoko-no-kawa" (the stream beside Menoyu), into which hot spring water from Ioyudani Onsen flows—the same source used by upstream facilities like Kirishima Hotel and Ryokojin Sanso. This stream contains milky white sulfur spring water with sediment of yunohana (hot spring deposits) at the bottom, attracting many hot spring enthusiasts who enjoy mud baths. The stream's temperature also changes seasonally, and bathing is possible when timing aligns.