The Lonely Mountain, or Erebor, was a mountain in the north-east of Rhovanion. It was the source of the river Running and a major Dwarven stronghold, the Kingdom under the Mountain, at the end of the Third Age and well into the Fourth.

The Lonely Mountain was possibly 3,500 meters tall, as it was snowcapped at spring. Geologically, it was rich in metals and jewels. The mountain was star-shaped with six ridges radiating as spurs from the peak. The south-western spur contained Ravenhill housing a Dwarven guard-post. Between the two western spurs was a narrow vale which was the exit of the Back Door, behind an overhanging cliff. Rough steps ascended to the top of the southern ridge along a narrow ledge turning east behind a boulder into a steep bay. Inside, the mountain was dug with passages and tunnels leading to cellars and halls and mansions such as the great chamber of Thrór near the Front Gate. A secret tunnel led to the "bottommost cellar". The main entrance into the mountain was the Gate of Erebor on the south side, opening onto a valley between two great spurs of the mountain. The River Running sprang from beneath the mountain and issued from the Front Gate, forming a waterfall that fell into the valley below. Inside the gate was a broad paved road that went alongside the river in a wide curve leading into the mountain. Not very far from the entrance was the Great Chamber of Thrór where feasts and councils were held. In the Lower Halls, there was a vast chamber called the Great Hall of Thráin at the root of the mountain. From there a secret passageway led to a hidden door in the western side of the mountain. The Back Door was invisible from the outside except on Durin's Day, when the light of the setting sun would reveal the keyhole.