The Brocken, also sometimes referred to as the Blocksberg, is a 1,141 m (3,743 ft) mountain near Schierke in the German state of Saxony-Anhalt, between the rivers Weser and Elbe.

The Brocken spectre is a common phenomenon on this misty mountain, where a climber's shadow cast upon fog creates eerie optical effects.

Today the Brocken is part of the Harz National Park and hosts a historic botanical garden of about 1,600 alpine mountain plants.

A little later, silica-rich granitic magma rose, some intruding into voids and gaps in the older rocks, but most being created by the melting of existing sediments.

For example, the summit of the Achtermannshöhe consists of contact-metamorphosed hornfels of the contact zone that, here, lies over the Brocken granite.

The alleged hardness of Brocken granite is not the reason for the height of the mountain, but the geological fact that it was well protected by its weather-resistant hornfels crest for a long time before erosion set in.

Today's blockfields of Brocken granite, as well as other rocks in the Harz National Park, particularly in the Oker valley, are therefore at least 10,000 years old.

Physical weathering, such as frost shattering, has played a key role in their formation, resulting in giant piles of loosely stacked rocks.

The mountain's summit is a subalpine zone with flora and fauna almost comparable to those of north Scandinavia and the Alps.

The Brocken is the only mountain in Germany's Central Uplands whose summit lies above the treeline, so that only very small spruce grow there and much of it is covered by a dwarf shrub heathland.

In the Brocken Garden, established in 1890, flora are nurtured by national park employees; visitors are allowed to view it as part of regular guided tours.

Some mammal and bird species that occur here are relics of the ice age, including the northern bat (Eptesicus nils soni), the alpine shrew (Sorex alpinus) and the ring ouzel.

The first documented ascent of the Brocken was in 1572 by the physician and botanist, Johannes Thal from Stolberg, who in his book Sylva Hercynia described the flora of the mountain area.

In 1736 Count Christian Ernst of Stolberg-Wernigerode had the Wolkenhäuschen ("Clouds Cabin") erected at the summit, a small refuge that is still preserved.

He also had a mountain lodge built on the southern slope, named Heinrichshöhe after his son Henry (Heinrich) Ernest.

Between 1821 and 1825 Carl Friedrich Gauss used the line of sight to the Großer Inselsberg in the Thuringian Forest and the Hoher Hagen mountain near Göttingen for triangulation in the course of the geodesic survey of the Kingdom of Hanover.

[9] A measurement carried out by the military staff of Prussia in 1850 found the Brocken's height to be at its present level of 1,141.1 metres (3,744 ft).

The tower continued functioning until September 1939, when the authorities suspended broadcasting on the outbreak of World War II.

To seal the area, the entire Brocken plateau was then surrounded by a concrete wall, built from 2,318 sections, each one 2.4 tonnes (2.6 short tons) in weight and 3.60 metres (11.8 ft) high.

As a protected area since 1939 and due to the decades of restricted access the unique climate of the Brocken provided outstanding conditions.

[12] The first record of a placename that resembles the present name of the mountain goes back, however, to the year 1176 when it is referred to as broke in the Saxon World Chronicle (Sächsische Weltchronik).

[13] Another early written reference to the mountain, this time as the Brackenberg, appears in 1490 in a letter from Count Henry of Stolberg.

In addition, animal and human sacrifices were offered by the Saxons to their supreme god, Odin, on the blockfields of the summit until they renounced them as part of their baptismal vows when Christianity spread to the region under Charles the Great.

[18] Another theory holds that the name "Brocken" is derived from bruch, a word used in northern Germany for bog or moor, which commonly used to be spelt as bruoch or brok.

The 100-kilometre-long (62 mi) Harz Witches' Path also runs from the Brocken eastwards to Thale and westwards via Torfhaus and Altenau to Osterode.

This section is a concrete slab track with a steady incline of about 20% and the runners are exposed above the tree line, often to a sharp, icy wind.

In early May each year the Braunschweig-Brocken Ultra Run takes place with 2 × 75 km (46.5 mi) legs spread over two days.

Brocken House (Brockenhaus), the modern information centre for the Harz National Park, is located in the converted "Stasi Mosque" (Stasi-Moschee), a former surveillance installation for the Ministry for State Security.

Technically poor and too small, it was partially demolished in 1912 and replaced with a large stone construction, the Hellman Observatory, that was not completed until the First World War.

In 1917 the academic and nature lover George Grobe took over running the observation post, his daughter supporting him until his death in 1935.