The Connecticut is one of only three Seawolf-class submarines, some of the most powerful and most capable submarines in the US Navy. Built at a cost of more than $3 billion each, these submarines were built to hunt Soviet submarines in deep waters.

So how exactly does a multi-billion dollar submarine equipped with high-end capabilities crash into an undersea mountain? A former US submariner talked to Insider about how something like this can occur.

"It's very rare for this to happen," Bryan Clark, a former US Navy submarine warfare officer and defense expert at the Hudson Institute, told Insider. "There's a lot of careful planning that goes into these operations."

"You do that careful planning to figure out what are the best maps or charts that we have of the area, what our plan is for where we're going to operate in terms of depth, what is the risk of there being an uncharted something on the bottom, whether it's a seamount or a pile of containers or something, and how to avoid areas that are likely to have those kinds of hazards," he said.

But sometimes unexpected contingencies force changes in plans, sometimes charts are not as good as they need to be, and sometimes sailors make mistakes.

The South China Sea is a challenging operating environment for submarines because it is very shallow, limiting the depths at which a submarine can safely operate with a low risk of being detected or running into something.

To make things more difficult for submarines, "the charts of a place like the South China Sea may not be nearly as detailed as you want," Clark said.

If a submarine is trying to be quiet and operate undetected, then it would likely be closer to the bottom and not relying on active sonar, which can alert the submarine to any potential dangers, such as naval mines, in its path but will also alert any potential adversaries to its position.

So in that situation, "you don't have anything that's looking forward of you in terms of active sonar," Clark said. "And, of course, you have no visual indication of what's ahead of you."

Submarines have passive sonar, but that only detects things that are making sound. "If you have something ahead of you that doesn't make any noise, like a seamount, you may not know it is there until you run into it," Clark explained.

"You might have your fathometer on, which is what you can use to measure the water depth below the ship. It has got a pretty narrow beam, so it's not as detectable," he said. "But the problem is that it only detects the depth underneath the ship and not forward of the ship."

That bottom-facing sonar could potentially detect inconsistencies in the charts and subtle changes in depth but may not necessarily get a vessel through a blind spot, where an uncharted topographical feature might be rising up sharply from the ocean floor.

"You could have a seamount pop up in front of you before your fathometer has a chance to detect it and you could run into it," Clark said,

He explained that these are challenges submarines face no matter how advanced they are. Submarines try to avoid these dangers by operating high enough in the water column, but sometimes that is not always an option.

It's unclear exactly what happened to the Connecticut, as the command investigation has yet to be released. The investigation has been passed up to the 7th Fleet commander, who will make any relevant accountability decisions. The sub is currently in Guam undergoing initial repairs.