Tsuchinoko Tankentai ~Myuu ga Ita Natsu~

"That summer, we met a miracle."

When Sei, Tsumugi and Kaichi set out to find the "mythic snake"
Sei already had a feeling that it was going to be a special summer.

Out of nowhere they met the strange girl Myuu, an equally strange
member of a secret society who went by "Sigma"
and a robot-plushie Dantes, all part of a research team.

Their curiosity piqued, Sei and friends chased the mystery,
not knowing that by summer's end, their encounter would bring about an
all-new capability for mankind......

Unknown beings. Aliens (?). A secret society-----The incredible self-discovery begins.

[From DLsite English]