Kyouka Jikken 3 ~Kurutta Bourei no Akume Idenshi Nyotai Kaizou Saishuu Shou~

The research facility sat before him. Here. Finally.
His mind shook with uncertain dread. After decades, the mystery he'd been solving was so close, but as he stood in front of the concrete structure, the taste in his mouth was something else.
In his hand, he held a letter. It described secret details of this facility where his father had come. It was addressed to his mother, who died when he was born.
The past... the letter... this strange place...
It would all come to a wild conclusion.

[From DLsite English]