Emptiness can surprise you. In general relativity, there is a solution where space itself gives birth to a black hole — literally from a point. It all starts at the moment when the event horizon (the surface from which not even light can escape) appears with zero radius and immediately begins to grow. It expands at the speed of light, like an inflating soap bubble, except this bubble is the edge of an abyss.
The trigger is a scalar field — an invisible substance permeating everything around. If its potential (the energy stored at each point) falls without bound, the field sets off the process. Far from the birthplace of the hole, an observer would notice a strange trace: a geometric defect, like a dent left after space has burst and collapsed into a horizon. A magnetic monopole, a particle with a single pole, could leave a similar trace.
This solution resembles the famous Schwarzschild metric (describing an ordinary black hole), but with one difference: here the mass is not predetermined; it appears and increases as the horizon expands. The starting point, the instant when the radius was zero, remains in the past as a scar on the body of the Universe. And this scar can be measured.
The scalar field in question resembles the Higgs field, but with one difference: its potential has no bottom, so energy can fall indefinitely, triggering the growth of a black hole.
In Peter Watts's novel Blindsight, alien ships manipulate scalar fields to create wormholes and control spacetime.