When I first embarked at this location everything seemed normal. However, there was a cave-in on embark. There was a fortress deep within the planet with a couple of entrances to the third cavern layer. The collapse occurred inside.
Thankfully below the masterwork sword
Regular, rare occurrence as I have discovered, but what was odd is that everything above the second cavern was normal, but between the second and third layers was 51 z-levels. Checking with the DF hack reveal tool, there was roughly one adamantine spire per embark tile within this 4x4 embark along with the fortress.
How to replicate:
1) Have a clean-of-mods raws folder.
2) Download the Hermit's united files and install them into the raws folder.
3) Gen up this world:
Created in DF v0.34.11.
[WORLD_GEN]
[TITLE:CREATE WORLD NOW = 3233333]
[SEED:VhuOwSOW32qlEDp4KIbL]
[HISTORY_SEED:vr7MIARsrUJl4IqlRlTI]
[NAME_SEED:mo5qzZXUpKGqf4Sc3DJN]
[CREATURE_SEED:ZplxxBpKL0AUU6RmO5IN]
[DIM:65:65]
[EMBARK_POINTS:1324]
[END_YEAR:125]
[BEAST_END_YEAR:125:-1]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:800:800]
[RAINFALL:0:100:101:101]
[TEMPERATURE:25:75:101:101]
[DRAINAGE:0:100:101:101]
[VOLCANISM:0:100:101:101]
[SAVAGERY:0:100:101:101]
[ELEVATION_FREQUENCY:3:2:0:1:0:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[MINERAL_SCARCITY:2500]
[MEGABEAST_CAP:4]
[SEMIMEGABEAST_CAP:9]
[TITAN_NUMBER:3]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:22]
[NIGHT_TROLL_NUMBER:11]
[BOGEYMAN_NUMBER:11]
[VAMPIRE_NUMBER:11]
[WEREBEAST_NUMBER:11]
[SECRET_NUMBER:22]
[REGIONAL_INTERACTION_NUMBER:22]
[DISTURBANCE_INTERACTION_NUMBER:22]
[EVIL_CLOUD_NUMBER:11]
[EVIL_RAIN_NUMBER:11]
[GOOD_SQ_COUNTS:1:16:0]
[EVIL_SQ_COUNTS:1:16:0]
[PEAK_NUMBER_MIN:1]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:4]
[VOLCANO_MIN:1]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:5:5]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:2]
[NON_MOUNTAIN_CAVE_MIN:3]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:10]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:264]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:136:272:136]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
4) Embark at this location (where the ruins symbol is at):