Why did they "search" for 4 days to find the missing sub when the Navy *heard it explode* and everyone knew it was diving on the Titanic?

Could someone please explain why they searched an area "twice the size of Connecticut" looking for the sub and spent four days searching the planet when the Navy HEARD the explosion when it happened, knew exactly where it happened, and everyone knew the sub was diving on the Titanic? Why didn't they just concentrate those resources right in that spot where the Titanic was from the start and find the wreckage the same day it happened? Something doesn't add up here and I have not heard any logical explanation.

Why did they search for 4 days to find the missing sub when the Navy *heard it explode* and everyone knew it was diving on the Titanic?
Updates
10 mo
So they weren't absolutely sure they heard a noise? That means they didn't go to the Titanic to find out even knowing that the sub was diving there exactly at the time they heard the noise? Makes no sense!

Lieutenant : "Admiral we think we heard an explosion in the area where that sub is diving on the Titanic at exactly the moment when they lost contact with the sub. But we're not sure."

Admiral: "Very well. Start a search area twice the size of Connecticut immediately!"
Why did they "search" for 4 days to find the missing sub when the Navy *heard it explode* and everyone knew it was diving on the Titanic?
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