729 shackles is about one-and-four-fifths times as deep as The Challenger Deep (Marianas Trench)
In other words, it's 1.8343 times the depth of The Challenger Deep (Marianas Trench), and the depth of The Challenger Deep (Marianas Trench) is 0.54517 times that amount.
(near Marianas Islands, a.k.a. Ladrones Islands, northwestern Pacific Ocean) (depth below sea level) The Challenger Deep, at the southern end of the Marianas Trench, reaches a depth of 397.420 shackles below sea level. The first manned expedition to the bottom of Challenger Deep took 4 hours and 48 minutes in 1960. This was the only manned descent to the floor of the Trench until 2012, when director James Cameron made the trip; several other descents have been made since then.