Bone Mountains

The Bone Mountains (often referred to as the Bones) are a massive mountain range in Essos, the largest in the known world, which completely bisects the entire continent of Essos - running the entire length from the southern coast on the Jade Sea to the northern coast on the Shivering Sea. The Bones are located east of Vaes Dothrak and the Dothraki Sea, the Red Waste, and Qarth - and are one of the major barriers to east-west land travel across Essos.

Geography
Historically the Bones have always been a major block to mass migrations and explorers, so that all of the lands east of them are only semi-legendary to people from Westeros. The Bones are the reason why the Dothraki and Qarth are the farthest east that people from the Seven Kingdoms have any regular contact with.

Men in Westeros know some vague details about lands and peoples east of the Bones, but they are semi-legendary to them. East of the Bones, roughly the southern half of Essos is dominated by the vast empire of Yi Ti, a lush semi-tropical land and ancient civilization, while to the north are vast plains dominated by the Jogos Nhai nomads. Southeast of Yi Ti are Asshai and the forbidding Shadow Lands.

The eastern side of the Bones used to be home to the great empire of Hyrkoon, and a few of its surviving city-states still survive, blocking the handful of mountain passes. The fierce warrior-women of Hyrkoon have smashed many armies and pillaging hordes that have attempted to force the mountain passes over the centuries. These city-states up in the inhospitable mountain passes include Bayasabhad and Samyrian.

In the books
The Bone Mountains haven't been mentioned in the main A Song of Ice and Fire novels, but were first introduced in the World of Ice and Fire sourcebook (2014). Their presence offers a convenient explanation for why the Dothraki and Qarth seem to be the eastern limit of the lands that people from Westeros are in regular contact with. A handful of explorers have ventured east of them to the Jade Sea and visited Yi Ti, but knowledge of the world east of the Bones drops off considerably, and the maesters caution that they only have a vague outline of these regions (geographically, socially, historically), much of which is only semi-legendary.

It is said that the Bone Mountains received their name due to all of the bones left littered in the mountain passes by armies and travelers who died there, some from wars but often just from the harsh environment itself. The Bones are of massive proportions, apparently taller than the real-life Himalayas, and extending across a much longer range. Given that they block east-west travel they are vaguely similar to the real-life Ural Mountains - though the Urals have more passes and are not nearly as inhospitable.

It is directly stated that despite bisecting the entire continent of Essos from north to south, there are exactly only three major passes through the Bones, making them truly a wall blocking east-west travel. The three surviving city-states of Hyrkoon on the eastern side of the mountains fiercely guard these three passes, apparently surviving on tolls they exact on merchant caravans. From north to south they are Kayakayanaya, Samyriana, and Bayasabhad. The Hyrkoonian cities are located far up in the mountains, and many armies of both the Dothraki and the Jogos Nhai have smashed against them, in vain. These three passes are the Steel Road (between Vaes Dothrak and Kayakayanaya), the Stone Road (between Slaver's Bay/Lhazar and Samyriana), and the Sand Road (between Bayasabhad and Qarth).

The northern sections of the Bone Mountains, between Kayakayanaya and the Shivering Sea, are the section closest to the Dothraki at Vaes Dothrak: they call this northern section the "White Mountains" due to their snowcapped peaks (Krazaaj Zasqa in their language). The earliest stories of the Dothraki seem to imply that during the Dawn Age their ancestors originally arrived at their current location by passing west through the Bones, but this is only conjecture.

According to Dothraki legend, the northernmost section of the White Mountains were once inhabited by massive "stone giants", whom the Dothraki called the Jhogwin - savage cousins of the non-human giants found in Westeros, but twice as large. According to the Dothraki, the Jhogwin severely dwindled ever since the Dawn Age due to inexorable encroachment by surrounding humans, and the last of them died out over a thousand years ago when they were massacred by the Jogos Nhai from the east after years of war. The exact dating of this event has not been confirmed but relies on vague oral tradition. Bones of the Jhogwin, however, have actually found at archaeological sites in subsequent centuries, confirming both their existence and their size (around 30 feet).