Hisa Masato's debut work: A black bear works for the mafia.

In the American Area 51, there is a society where many varied supernatural creatures—kappa, vampires, aliens, gods and goddesses of various mythologies, mythological figures (like King Arthur), and more—make their home. In addition to the non-humans, there's also a population of humans, people who can't live in the ordinary world for one reason or another. One of these people is Magoi Tokuko, commonly known as McCoy, a tough woman who works cases with the help of her kappa associate, her deep knowledge of the area, and her pistol. In her office, which she's built in an unfinished subway station, she takes on all kinds of jobs (recovery of stolen goods, infiltration, etc.) from the many humans and non-humans who come to pay for her help.

(Source: MangaHelpers)

One of the Japanese promos calls it "Da Vinci Code meets Jurassic Park".
I think that sums it up pretty nicely. It's got dinosaurs, spies, intrigue, action, and conspiracy, all rendered in lovely film noir-esque art with a little fan service cherry on top (Lily!).
And for the slightly geeky, it's got cool (and sometimes obscure) references to art, literature, film, history, science... you name it. Definitely a manga for adults!
[From MU]

Shio Ogura is a Japanese high school student, who is visiting Taiwan on a school trip when she is suddenly attacked by monsters. Agents known as "E-Gene Holders" from the government agency DOGOO also arrive, who wield weapons infused with the spirits of historical figures. Shio is revealed to also be an E-Gene Holder when the soul of Oda Nobunaga awakens after she tries to rescue a friend.

The hero, Kamuya Ride, stands on the boundary line between the god and human worlds. He's the only one who can stop a mysterious man from awakening the previously sealed Earthly Deities from their 200 year slumber. Alongside Kamuya Ride is the Prince of the Yamato, a powerful bloodline responsible for the Earthly Deities' fall from power years ago. Can these two close the Earthly Deities' gates into the human world and staunch the growing rebellions in the land?

(Source: Mangadex)