Buried Treasure is a divination card. A set of three can be exchanged for a random Sulphite Scarab.
Scarabs are a type of map fragment. They do not open portals to new areas but are instead used to augment existing areas with various effects.
Scarabs are primarily found within the Safehouses of the Immortal Syndicate in the Intervention department, but on very rare occasions, a Syndicate member can offer you a random Scarab in exchange for their freedom upon defeat. The quality of Scarabs depends on the rank of the member.
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Buried Treasure can be dropped in the following areas:
Crystal Ore Map • Dig Map • Excavation Map • Flooded Mine Map • Geode Map
The fast way to get Buried Treasure is by exchanging currency items such as Chaos Orbs.
Someone posted here on Reddit that it drops in Excavation. I don’t know what deep delve is for you, but delve is a lot harder now with the buff to rare and magic monster life, I’m having a hard time going back to depth 1500 on std with the same char, at that depth rare have like 20 time shaper life now.
GGG has a sense of humor. Farmers of the Quarry will have flashbacks.
Not all that surprising to be honest. Most likely drop in Quarry too. The card might drop in other maps as well, but at least it dropped here, just a random pack of monsters.