Keya no Oto (芥屋の大門).
I write this as a solo travel experience.
It is located in Itoshima, Kyushu, Japan. The place is not too far from futamigaura torii. You can go from one to another by public transport (bus). If you like unique natural places you should come by.
Here is the place if you want to get on the boat to see the closer place
this. Although they also have schedule on weekday, but it is my suggestion that you come on weekend in terms of a passenger number. Also do not forget check their
instagram first if you want to come, because when the weather is not too good, the boat will not go.
Nanatsugama (七ツ釜). I went there with a friend. There are two ways to go on boat. I will explain each of them. (1) This way is better if you will go with a friend or by car, because you have to walk around 2 KM or 30 minutes on foot from the entrance public transportation (bus). You will stop at the nanatsugama entrance bus stop and go until
here. This way is not only you can go on boat, but also went up to the whole area of nanatsugama. I choose this way better, but if you will go alone i will suggested the 2nd way. For the information of the boat, you may check
here (2) This way you can go along with the bus close to the ship dock and big city. You may check the information from
here
These are two places that are known as the three largest basalt caves/formations in Japan. The difference between those two is, keya no oto only have small 'cave' but big, while nanatsugama contain 7, but only two caves that ships can enter. If you have time, I suggested to get into those two, but if I should choose, nanatsugama is way more worthed, but for the easier transport you may only go to keya no oto.
