Overview
SunkenCrypt4 is a Valheim world location that spawns in the swamp biome, identified internally as SunkenCrypt4. World generation places up to 400 per world, with terrain constraints (max delta 4, altitude 0-2), and an exclusion radius of 12 (14)m, which is why some seeds have far more of them than others. The data tags it as prioritized.
About
SunkenCrypt4 is the largest, prioritized Sunken Crypt dungeon variant in the Swamp. Sunken Crypts require the Swamp key from The Elder to open, and inside they contain Muddy scrap piles that yield Iron scrap.