By its very nature, the black hole at the heart of our galaxy is impossible to spot.
Its overwhelming gravity allows nothing to escape, not even light. Massive enough to send shivers through space-time itself, yet perfectly invisible, it lurks in the darkness like a monster from a child's nightmare - felt but unseen.
It is the stuff of physicists' wildest dreams.
"Black holes are basically the most mysterious objects in the cosmos," said Shep Doeleman, an astronomer at the Harvard-Smithsonian Centre for Astrophysics. Even Albert Einstein almost didn't believe they were real, even though it was his theory of general relativity that helped predict them more than 100 years ago.
In the century since, scientists have been able to sense black holes through observations of their influence on nearby matter. And with last year's detection of gravitational waves emitted by two colliding black holes, they've also heard them.
But no one has ever seen a black hole. The enigmatic objects hide behind an "event horizon" - the boundary at which gravity acts like an invisibility cloak, wrapping around light and matter and swallowing them whole. No telescope on Earth is powerful enough to penetrate that abyss.
At least, no single telescope is. Today, a battalion of 120 astronomers working at eight observatories on four continents will mobilise in an unprecedented effort to image the black hole at the centre of the Milky Way, a body named Sagittarius A*.
By combining observations from points across the globe, they'll create a virtual observatory the size of Earth itself. The "Event Horizon Telescope," they call it.
If all goes according to plan, the EHT should capture the dark silhouette of Sagittarius A* against the hot, glowing material that surrounds it, offering the first-ever glimpse at a black hole's event horizon. The resulting snapshot could confirm our understanding of the laws of the universe - or upend it.
"It's a very bold and gutsy experiment," Stanford University theoretical astrophysicist Roger Blandford, who is not involved in the project, told Science last month. "It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing."
Sagittarius A*, like other supermassive black holes, was probably formed during the earliest days of the universe. It is about four million times as hefty as the sun but only 18 times as large, and it sits on space-time like a bowling ball on a trampoline, warping everything around it.
Under these extreme conditions, the laws of physics are pushed to their limits: Gravity is twisted; light is torn apart like taffy; matter is made to vanish from existence. Doeleman, the EHT's director, calls black holes "the most exotic animals in the cosmological zoo."
Studying what they eat and how they behave can teach scientists a great deal about the workings of the universe, he said.
Over five nights during a 10-day window beginning today, the eight telescopes of the EHT will swivel as one toward the centre of the galaxy.
Scientists in Chile's Atacama Desert, on a volcano top in Hawaii, at the frozen expanse of the South Pole and in the high, dry mountains of Arizona, Mexico and Spain will be ready to catch the data. If conditions are exactly right at each location - skies clear, instruments working - then Doeleman will give the signal to start observing.