Cloud Plane [1]

When they arrived back on earth, Damien wasted no time. Their group immediately headed for the Asgard headquarters that stood tall in the middle of Los Angeles.

"The mana density of the atmosphere has dropped intensively," Rose commented as they were on their way. 

Damien couldn't really feel the difference since the planet itself provided him mana as long as he was present on its surface, but when he actively attempted to tell the difference, he noticed that around 50% of the ambient mana in the world was gone.

'This must be the work of those old men, so they should be ready to leave.'

While communicating with the world core, he was able to tell that it didn't really mind the drain of mana that occurred. It could simply produce mana to make up for the difference, even if it took many years to do so.