File:Lowest possible block in a Custom world.png

Summary
This is the lowest temporary block I managed to place in a Custom world, at which point the game was unplayable for me. Update if you achieved a lower value with your better PC to decrease the upper bound in the article. Custom settings used: {	"bonus_chest": false, "dimensions": { "minecraft:overworld": { "type": { "logical_height": 65856, "infiniburn": "minecraft:infiniburn_overworld", "effects": "minecraft:overworld", "ambient_light": 0.0, "respawn_anchor_works": false, "has_raids": true, "min_y": -65536, "height": 65856, "natural": true, "coordinate_scale": 1.0, "piglin_safe": false, "bed_works": true, "has_skylight": true, "has_ceiling": false, "ultrawarm": false },			"generator": { "biome_source": { "seed": 1, "large_biomes": false, "type": "minecraft:vanilla_layered" },				"seed": 1, "settings": "minecraft:overworld", "type": "minecraft:noise" }		},		"minecraft:the_nether": { "type": "minecraft:the_nether", "generator": { "biome_source": { "preset": "minecraft:nether", "seed": 1, "type": "minecraft:multi_noise" },				"seed": 1, "settings": "minecraft:nether", "type": "minecraft:noise" }		},		"minecraft:the_end": { "type": "minecraft:the_end", "generator": { "biome_source": { "seed": 1, "type": "minecraft:the_end" },				"seed": 1, "settings": "minecraft:end", "type": "minecraft:noise" }		}	},	"seed": 1, "generate_features": true }