"The pin-drop moment when we realised that the mystery result in our equations was telling us that the black hole we were studying had a pressure – after months of grappling with it – was exhilarating," Kuipers said .

It's unclear what's causing the pressure, and according to the team's calculations, it's very small. Moreover, it's negative – expressed as -2E^-46 bar for a black hole the mass of the Sun, compared to Earth's 1bar at sea level.