What is the main difficulty in bringing up the valuable nodules located on the bottom of the Pacific Ocean?

The main difficulty in bringing up the valuable nodules located on the bottom of the Pacific Ocean is that they lie in water so deep they cannot be easily retrieved. The ocean's depth presents significant technical challenges, including the extreme pressures found at those depths, which require specialized equipment to operate safely and effectively. Traditional mining techniques used for shallow deposits are not applicable in such deep waters, making extraction complex and costly. The depth also complicates logistics and transportation when it comes to mining operations.

Other options suggest issues related to visibility, damage during retrieval, or the notion that there are no difficulties in mining. However, the primary and most substantial barrier is indeed the depth of the water where these nodules are located.