Ryusendo Cave

Ryusendo is one of the three great limestone caves of Japan, with about 3,600 metres surveyed and an estimated total considerably longer, of which around 700 metres are open to walk. What sets it apart is the water: several underground lakes fill the lower chambers, fed by rain filtered through the limestone, and the transparency has been measured at over forty metres, among the clearest fresh water anywhere. They are lit from below, so the third lake - the deepest accessible at about 98 metres - reads as a shaft of blue going down out of sight. The cave is also home to five species of bat and is designated a natural monument partly for them. The temperature inside holds at about ten degrees year round, which makes it a relief in August and cold in every other month. A separate cave nearby houses a small museum of the excavation.

What you actually see

• The underground lakes - transparency measured at over forty metres.
• The third lake - about 98 metres deep, lit from below as a blue shaft.
• Bats - five species, part of the reason for its natural monument status.
• Ten degrees year round - cold in every month but August.

When to go

All year. Two hours. Summer for the relief of the cold air.

Honest expectations

The walkway is wet, steep in places and has many steps. Only a fraction of the surveyed cave is open. Iwaizumi is remote and buses are infrequent. The cave has closed after flood damage in the past.

Nearby

The Sanriku coast is east; Morioka is west.