Players who stumble upon the Underground Passage in Baldur's Gate 3 will likely use it as a convenient route between the Druid Grove and the Goblin territory, just like the Goblins themselves. However, there is a hidden treasure waiting for those who explore further after defeating any enemies.
However, locating the chest is much easier compared to finding the key. Many players in Baldur's Gate 3 lack locksmith skills, so obtaining the Charred Key to unlock the Heavy Chest may prove challenging. It is not located in a place that most players expect to reach.
Getting The Treasure In The Underground Passage
Disable the Guardian Statues.
Jump to the middle stalagmite.
Locate and loot the Dead Human for the Charred Key.
Jump back to the main area.
To access the Heavy Chest, make your way through the south entrance and stick to the right wall.
Priority should be given to rescuing Findal and addressing the dangerous Guardian Statues. Once those matters have been dealt with, search for the Charred Key on the corpse of a deceased human. Reach the location by leaping onto the sizable stalagmite positioned in the center of the cave. You will find the body at the foot of this rock formation.
Once you've hopped back, all that's left is to pilfer the Heavy Chest. If you approach from the south, stick close to the right wall and you won't be able to miss it. For those who entered from the opposite side (maybe those who set Sazza the Goblin free from her makeshift prison), the chest is located on the upper level, right by the left wall as you ascend or take the ramp towards the southern exit.
Baldur's Gate 3 is currently in Early Access for PC and will release on August 3rd, 2023. The PlayStation 5 version will launch on September 6th, 2023, while the Xbox Series X/S version is currently being developed.