To me, the book felt kind of like fragments or stories.

With the background of a future Earth impacted by climate change, overpopulation and related issues, we're shown pieces of a story about a black-hole-like object created by humans that ends up in the Earth's core - posing a threat to the planet. Complications arise when those trying to save the planet have to deal with the less idealistic motives of governments and monied interests. There is also the question of possible involvement of hostile aliens.

Along the way, the "stories" make up several plot threads with some recurring characters.

I have mixed feelings. The book provides speculation about the future with climate change, population / natural resource issues, etc. It speculates about extraterrestrials, harmful and helpful. It has ideas about singularities (point-like masses) and gravity. It has ideas about government spying, plutocratic conspiracy, computer hacking, etc.

It has an afterword in which Brin tries to clarify what was established science, what was speculative science and what was pure fiction.

On the other hand: For quite a while at the beginning the plot threads seem jumbled. Some of the plot threads never really join the central plot, although they provide dialog and such to present ideas. The central plot turns into something approaching military SF. The book is on the long side. And, although I don't assume all aliens must be friendly, aliens attacking Earth is too common a theme and plays too much on a dangerous side of human psychology - so I generally don't find such themes that satisfying.

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