The correct answer to this open question is the following.
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The main obstacle to exploring the ocean depths is the excessive pressure the sea exerts on humans and all the equipment needed to explore the depth of the oceans and the corrosive effect of the water.
The pressure is too dangerous for humans because it has a direct effect on the lungs of humans.
Another costly reason is the power to turn the equipment needed to explore the depth of the sea. Power resources are too expensive and their duration, so limited.
