The Pulse-Ignition Thaumnode, aka the P.I.T, or the Pit. explanation.(E)

Created(E) by (or with the help of) Hindenburgh, a great great uncle of Sylvester, presumably a son of Ernest and brother of Philbert.

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 rather spectacular(E).

Mortimer finds it creepy(E), perhaps because magic flows through him much more strongly than most.