missing titan submarine

Why is finding a missing submarine in the ocean like 'looking for a needle in a haystack'?
On June 19, OceanGate Expeditions announced that their submarine had suddenly disappeared at sea with 5 expedition members.
3 hypotheses about the missing Titan submarine in the Atlantic Ocean
According to Chris Parry, a former Royal Navy officer, the submarine may have lost connection, but it is possible that an accident occurred while passing through the wreckage of
Titan submarine crushed, 5 people on board died
The US Coast Guard said debris recovered showed the Titan submarine had been sunk.