The Lost Mound Unit of the Refuge is the old Savanna Army Depot.
The area contains about 7,000 acres of uplands including the largest remnant sand prairie in the state, and 5,500 acres of bottomlands and is listed as a statewide significant natural area by the Illinois Natural Areas Inventory. At least 47 Illinois endangered and threatened plant and animal species have been observed at the Depot.
The unit derives its name from a prominent geologic feature, Lost Mound, located in the adjacent bluffland.
The Depot was used as an artillery test firing site from 1918-1919 and as an ammunition recycling, storage and testing facility from 1919-2000.
Loggerhead shrike and grasshopper sparrows have been seen on the prairies of the Lost Mound Unit.