The Lost Grounds (喪失の地, Sōshitsu no chi (ロストグラウンド, Rosuto Guraundo)) are special areas within The World that have existed since Fragment. This title is first used in The World R:2. Lost Grounds seem to be synonymous with Sacred Zones (聖地, Seichi), areas based on the story of the Epitaph of the Twilight. They may or may not be related to the Legendary Lands, areas related to the story of The World. One of the Lost Grounds, The Creator's Room, is also known to be one of Harald's Rooms. The Japanese festival-themed area Δ Hidden Forbidden Festival matches the "Hidden Forbidden" naming pattern most Lost Grounds share, but it is unknown if this area is also a Lost Ground. Piros the 3rd hypothetically cites that a certain graphics designer designed some, if not all of the lost grounds as his reason to protect their graphics from Tri-Edge's signs.
- Arche Koeln Waterfall
- Balbol Museum
- Briona Gwydion the Dragonbein Range
- Coite-Bodher Battlefield
- Dark Morrigu Barrow Wall
- Dead World of Indieglut Lugh
- Great Temple of Caerleon Medb
- Hulle Granz Cathedral
- Hy Brasail, the Isle of Kings
- Inverted City Megin Fi
- Morrigu Barrow Wall
- Six Ringing Peaks of Al Fadel
- Sugar Mansion Sif Berg
- The Creator's Room
- Wailing Capital Wald Uberlisterin
- World of Sin Ran Hati
- In .hack//G.U., the music that plays within the Lost Grounds is in a fictional language unique to the series; however, the background music in Hulle Granz Cathedral is German.
- Dark Morrigu Barrow Wall (Θ Returning Invincible Huge Being) is the only Lost Ground that is accessible from a Chaos Gate using a phrase not starting with Hidden Forbidden.
- World of Sin Ran Hati, also known as the Forest of Pain, is the only Lost Ground that is intended to contain non-boss battles by CyberConnect Corporation.