Too bad Street Magic removed a notation that was in SR3's Magic In The Shadows regarding projection through the Earth. Earth elementals/spirits could add their Force as a DP bonus to how quickly you could move through the Earth, and guide you without the risk of getting lost. IIRC being a drake of one of the Great Dragons (Lung, I believe) would let you freely astrally project through the Earth as if it wasn't there.
As for the suggestion of the Marianas Trench, you really wanna tread where Lung, Ryumyo, and/or the Sea Dragon are likely to claim as their domain? Yeah, yeah, I know the Sea Dragon's supposed to primarily lair near Wales, but when she claims the seas as her domain you seriously think she's gonna limit her self to just the Atlantic? And the trench isn't just a deep pit at the floor of the Pacific but a fault between the Philippine & Pacific plates, part of the Ring Of Fire that Ryumyo & Lung are vying for control of.