BEIJING—China plans an ultradeep dive by a manned submersible beneath the Pacific that would propel it past the U.S. in a race to explore potentially vast mineral resources in the deepest parts of the world's oceans.

The Jiaolong—named after a mythical sea-dragon—left China on board an oceanographic research ship on July 1. It arrived on Saturday at its destination in the northeastern Pacific, between Hawaii and North America, where it is to attempt a dive to 5,000 meters, or about 16,400 feet, according to state media reports....