The U.S. has officially expanded its geographical territory by one million square kilometers -- an area nearly 60 percent the size of Alaska.
The expansion is based on internationally established definitions of the continental shelf, and comes from research and surveys conducted by various groups going back to 2003, confirming where the continental shelf actually is.
Interestingly, this doesn't include the water column above this territory, so it doesn't mean control of fishing or shipping lanes. Only seabed/underground mineral/drilling/pipelines control. Depending on policy and which political party is in control at the time, this could mean preventing others from drilling these areas, or (more likely?) making a profit off allowing drilling here.
Most of the addition is in the Arctic, but includes territory around the whole country.
Their claims are based largely on building artificial islands which extends their 200 miles from the coast claims (by aging more coast). This US claim isn't extending coast (so no control of the water column or surface), only continental shelf and mining rights