<a href="https://en.wikipedia.org/wiki/Rotenburo" class="extiw" title="w:Rotenburo">Rotenburo</a> (open air bath) of Hotel Urashima in Nachikatsuura, Wakayama prefecture, Japan. Facing the Pacific Ocean, it is located inside a large natural cave that has been eroded by the sea.
<a href="https://ja.wikipedia.org/wiki/user:663highland" class="extiw" title="ja:user:663highland">663highland</a>
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This is a complete original image ( no retouch, and no re-compression ).