I hope Sasano carries the location of that lake to his grave.
The Sci-fi element to this is fascinating, I thought it'd be an interesting series of vignettes about festivals around Japan and their historical/mythological roots, but an overarching narrative that it's all derived from an ancient alien civilization is very, very intriguing, especially with the interwoven dual narrative of the present-day researching seeking the truth while protecting the legacy & ore of those people, and the revelations of the festivals and the characters involved/connected to the larger web of tradition, both real and "created", pervading it all.
Truly cool stuff, and with lovely highs and lows of tone and mood throughout.