Sundern, Protected area in Hemmingen, Germany.

Sundern is a 50.4-hectare nature reserve (NSG HA 004) in Hemmingen, south of Hannover, protecting a silted-up meander of the Leine's glacial valley. Wet woodland and floodplain forest remnants cover most of the old loop, while drier ground carries oak-hornbeam and beech-oak stands with much old timber. Fields between the woods are farmed.

The area received protection on April 5, 1931, making it one of the earliest designated nature reserves in the Hannover region of Lower Saxony. This conservation decision reflected growing awareness about preserving natural habitats during the first half of the 20th century.

The mausoleum in the reserve was built in 1842 for Carl von Alten, a Hanoverian-British general and statesman. It was designed by court architect Georg Ludwig Friedrich Laves and completed in brickwork by architect Conrad Wilhelm Hase. It is considered the first Neo-Gothic brick building in northern Germany. A preservation society was founded in 1987 to save the structure from ruin.

Visitors should keep dogs on leashes and stay on marked paths to protect local wildlife and habitats. The trails are well-marked and accessible for walkers and cyclists of different fitness levels.

The mausoleum of Graf Carl von Alten, built in 1842 for the Hanoverian-British general and statesman, stands within the reserve near Wilkenburg. Designed by court architect Georg Ludwig Friedrich Laves and finished in brickwork by Conrad Wilhelm Hase, it is considered the first Neo-Gothic brick building in northern Germany. It decayed into a ruin, prompting the founding of a preservation society in 1987.