CHICAGO (NewsNation) — A Canadian aircraft searching for the missing Titanic submersible detected “underwater noise” on Tuesday from the vicinity of the location where the crew was touring the wreck site.

The crew searching for the sub heard sounds every 30 minutes and again four hours later after additional sonar devices were deployed, The Independent reported. However, after deploying an underwater robot to investigate, the Coast Guard said they didn’t find anything.

But why is the “underwater noise” significant?

Retired U.S. Navy Capt. Brad McDonald said there are a lot of sounds in the ocean, but a metallic sound made by a possible banging by another piece of metal will travel miles and miles underwater.

“In the old cat and mouse games that submarines play, being the quietest submarine is usually the way to win the game,” McDonald explained. “And you can have the quietest submarine in the world. But if a sailor drops a wrench in a bilge, that noise will travel for miles.”

McDonald said when a metallic noise is picked up on sonar on a Navy submarine, it’s very clear to the crew it’s most likely another submarine.

So, why is it so hard to locate the missing Titanic sub?

While McDonald said he doesn’t know a lot about the Titan submersible, he said he would have suspected it to have a pinger that would help rescue crews find its location in accident situations. However, it has not been confirmed if the vessel has one.

On top of that, he explained that looking for this submersible is like looking for a needle in a haystack. It took decades for crews to find the Titanic, and this specific vessel is much smaller, he said.

“A typical U.S. Navy submarine would have no capability to find this thing,” McDonald said. Despite what people have speculated, a Navy submarine would not be able to help discover the submersible.