Build underground lost-city megabas...

Build underground lost-city megabase (NOT modern). Deepslate + prismarine. Vertical aqua-core tower (19–23 wide) with glowing water core (7x7/9x9). 5 floors: Trophy/Armor, Quarters/Map, Enchant+Forge (15 books), Brewing, Main Storage. Base has lock-chamber with prismarine rune inlays leading down to library vault. Below: rectangular mini megavault (31x21, 10–13 tall) cathedral deepslate arches/ribs + prismarine circuitry. Rooms: main library hall w bridges/catwalks; enchant annex; archive storage; scribe room (lecterns+cartography+anvil/grindstone); hidden reliquary behind fake shelf. Cyan glow w hidden sea lanterns. Expandable tunnels.