In this video from Grenoble, France, two teams of people attempt to separate the halves of a metal ball. Spoiler: The ball wins.
There are no magnets involved, nor are the two sides of the ball locked to each other with any physical device. Instead, what you see here is a classic, hands-on physics demonstration that's been a crowd-pleasing favorite since 1654.
Magdeburg Hemispheres is the fancy name for the two halves of a hollow metal ball that you see in this video. You hold them together, hook them up to an pump, and suck out all the air from the cavity in between. What you're left with is a vacuum … and a great way to show people the power of atmospheric pressure. At Skulls in the Stars, Greg Gbur explains:
All objects within the atmosphere are under constant bombardment from air molecules traveling every which way; this atmospheric pressure is not noticeable to us because our bodies have an internal pressure that matches and balances it.
When the hemispheres are first placed together, the air pressure within them balances the air pressure outside, and they are easily pulled apart. When air is removed from the interior of the hemispheres, however, there is no longer any force pushing outward: the atmospheric pressure outside dominates, pushing the hemispheres together and keeping them from being separated.
In the original Magdeburg Hemispheres demonstration, teams of horses couldn't separate the vacuum sealed ball.