The Solaceon Ruins are in Solaceon Town.
You can't catch a Lugia anywhere in Pokemon Diamond.
In Solaceon town. There is cave in east part of town.
go from violet city to the uknown caves. solve the sliding puzzle(a kabuto) you'll fall into the cave. the uknown should appear
turn back cave
I beat the league on Pokemon Diamond version, and there was no cave involved, if that's what you were asking.
You cannot find the Pokemon Darkrai in Turnback Cave in the game Pokemon Diamond. Instead you can find him in Newmoon Island.
She is in wayward cave.
wayward cave
You can get the TM28 (Dig) on Route 214 in the Ruin Maniac Cave in Pokemon Diamond.
you can get it from fire red in cerulean cave in mirgrate it to diamond
brake a leg
in wayward cave