There are some cool stuff in the deep (>100’ / 30m), but rarely compare to the shallow (<60’ / 20m). In the deep, it’s cold, dark (you need a good dive light to see better down there), short NDL bottom time to deal with. You need some experience, good air consumption (what I mean by a good air consumption is when you run out of NDL before you run out of air, e.g., SAC of 0.4 cfm, comes with experience), cool under pressure when you encounter problem (again comes with experience), a good buddy (in my case a guide) to watch me for sign of narcosis & to discuss gas planning, etc.
I like to see fish more than wrecks and did find 2 cool fish in the deep (2 out of 700 dives is what I meant by rarely). The 1st one is oceanic mola-mola at 144’ (43m) in Nusa Penida, Bali, in 2012, see the picture, below.
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The 2nd one is Zebra shark at 147’ (44m) in Palau, in December 2017. See the picture below.
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The best one is to go to 1000’ (300m) deep without getting wet. That’ll be in Cocos this September. I can’t wait.