Yep... glad it's not Concorde!
However, to completely submerge a 747 with a tail height of just under 20m, plus leaving room for boats to be able to pass safely without grounding on the tailfin, I reckon you'd have to drop her to 30m.
That's in the realms of the PADI Advanced diver! I'm a qualified open water diver and good for 18m. Resort divers are lucky if they get much beyond 9m.
I wonder how long it would remain looking like a 747? You'd be surprised at how quickly marine ecology will sprout up over a new 'reef'!
And just think of the ecological uproar if they didn't drain it of all hydraulics, fuels etc!
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