The Pulse-Ignition Thaumnode, aka the P.I.T, or the Pit.
The Pit originally boosted airships on their way to the Far Eastern Shore. It was mostly wrecked in the crash, but the broken remains still occasionally sends a beam of magic ("pit flares") into the sky. Stories from before The Crash indicate it was .
Mortimer finds it , perhaps because magic flows through him much more strongly than most.