This WWII memorial is marked on Google Maps as 'Cenotaph for Former Ibusuki Naval Air Base Site (指宿海軍 航空基地 哀惜の碑)'. It's built on top of an old air raid shelter and is a bit tricky to spot in the parkland. It is a memorial to the 82 suicide pilots who took off from the nearby air base from April to June 1945.

There are some information boards entirely in Japanese, however it appears the planes used were slow reconnaissance float planes, some or all of the pilots were young, some still teenagers. The fate of the pilots and their planes is unknown, I believe the planes were only given enough fuel to reach their target (Okinawa), and I gather the memorial praises their sacrifice to protect Japan.

I'd prefer these types of war memorials also inform the modern public of the pointless waste of life at that stage of the war, directed by old men who were trying to avoid the rapid approach of their own demise.