Portsmouth Mine Pit Lake, sometimes called the Portsmouth Pit, is the deepest lake completely within the state of Minnesota, USA. It has a depth of over 450 feet (137 m), according to the most recent Minnesota DNR data. Lake Superior, over 700 feet deep off the north shore of the state, is technically deeper. The 120-acre (49 ha) artificial lake is a former iron mining pit in the Cuyuna Range that has since filled with water.

This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (June 2020)

Quick Facts Location, Coordinates ...

Portsmouth Mine Pit Lake
A view of the swimming area from the campground on the west shore of the lake.
Portsmouth Mine Pit Lake Show map of Minnesota Portsmouth Mine Pit Lake Show map of the United States
Location	Crosby, Minnesota
Coordinates	46°29′22″N 93°57′53.5″W
Type	Artificial lake
Basin countries	United States
Surface area	121 acres (49 ha)
Max. depth	450 ft (140 m)

Close

It is part of the Cuyuna Country State Recreation Area, located in Crow Wing County. The lake is within Crosby city limits.

The Minnesota DNR has repeatedly stocked the lake with brook and rainbow trout.

On August 19, 1957, the mine pit, then not yet filled with water, was launch site for a big stratospheric balloon, the second flight of the Air Force's Project Manhigh.

• Details of the Manhigh II flight
• MN DNR Lake information - Portsmouth Pit
• MN DNR MN Facts - Lakes, rivers & wetlands 2003