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https://tibia.fandom.com/wiki/Foreigner_Quarter - Open Areas, Room with 3 blood Crabs —
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https://www.tibiaqa.com/24113/is-it-possible-to-open-the-door-from-the-repenters-quest door from Repenters Quest
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https://tibia.fandom.com/wiki/Deepsea_Blood_Crab - Room under Rookgaard Academy
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https://www.tibiaqa.com/21972/is-it-possible-to-enter-or-see-whats-inside-the-hole-fabulous-items-tibia?show=21972#q21972 - Fabulous Items Tibia Cave
-Room above NPC Gamon
Calcanea-
https://tibia.fandom.com/wiki/Calcanea
from TibiaWiki which has been mentioned by Xarkost:
Mainland
Barbarian Arena 4th reward room - The Barbarian Arena has an unreachable reward room.
Peninsula between the Edron Troll-Goblin Peninsula and the Edron Orc Cave - This is a sizable plot cut off by a hill. Map
Troll hole - Down this hole southeast of Thais is a not unlockable door with trolls in a room behind the door. Map
Basilisk Room - This is an unreachable room that can be seen from the Slime cave in the city mine of Kazordoon. Map
Colossus Fortress - The Kazordoon entrance has a large unreachable area.
Kazordoon Fortress - Inaccessible area that connects the Kazordoon entrance and Dwacatra gates.
Shadowthorn - In the Time Ring Quest room there are magic walls blocking a passage. Some people say it is quest related. Map
Another passage blocked by magic walls can be found at the Spike Sword Quest. Behind it, you can see and hear dragons. Map
Kha'labal - Slightly west of the mountain pass between Darashia and Ankrahmun is a wall built into Kha'Zeel. No one has discovered how to get past this wall. It is commonly called the "spectral tomb". Map
Poisontower of Edron - This tower lies northeast of the Edron Dragon Lair and directly over a passage in the Hero Cave. but has not been reached by anyone who wishes to tell about it. Some people say this tower is quest related. Map
Edron ghoul cave, sealed room - Here, in this cave is a sealed room with a switch inside and it is not known how to enter this room. Map
Northern Green Claw Swamp - There is an unreachable hole in the ground and there is a cave underneath that can be seen from the three witches' cave. Map
Venore's jail has a special holding cell which you cannot access and its linked to a dead body on the way to venore's dragon lair, that's supposed of someone who was trying to escape from prison and had the roof falling on him, you can also see a spoon that could have been used to dig the tunnel, the entrance is here. Map
Giant Spider Tower inside of hellgate. Map
A small room with a Demon under Liberty Bay. Map
Isle of Solitude - Map
Island northeast of Senja (called Banor's Eye) Map
Calcanea - Map
Rookgaard
Ghoul - An unreachable ghoul near the Katana Quest room. Map
Minotaur Mage Room - An unreachable room with a miotaur mage in Minotaur Hell. Map
Seagull Mountain - Mountain with a seagull south of the premium account area. Map
Sword of Fury Isle - An unerachable isle with stones, fire fields and a spike sword. Map
Source:
https://tibia.fandom.com/wiki/Unreachable_Locations