The Iwate Nippo site reports on a brewery with a "natural refrigerator" used to mature their sake at low temperatures.
Iwate is famous for the Ryūsendō limestone cavern, one of the three largest in Japan, found near the town of Iwaizumi - which is also home to the Senkin sake brewery. This is the fourth year that they have laid down sake in the caves to let it mature.
The brewery laid this year's sake down on 16 May 2018, continuing their experiments with storing sake brewed with the cave water in the cave itself. 10 members of the brewery staff prayed for safety at the entrance to the Ryūsendō tunnel, then started moving 700 1.8 L and 1,570 720 ml bottles of Ryusen-Yaezakura junmai ginjō to a storage area 230 m inside the cave.
The sake will be left to mature for four months, ready for release in mid-September when it will go out to retailers in the towns of Iwaizumi, Morioka and Miyako in Iwate Prefecture, as well as the brewery's own shop and the local Michi no eki roadside regional speciality market. The 1.8 L bottles will retail for JPY 3,000 and the 720 bottles for JPY 1,500 (excluding tax).