As soon as you have surfing skills
Pokemon Sword and Shield has a ton of layers below the surface, and many of them are based on the Wild Area.
As soon as you have the opportunity to surf (along Route 9 downstream of Circhester, as part of the story), you'll want to head to Lake Outrage in the Northern Wild Area.
To get there, all you have to do is go to the Northern Wild Area, just below Hammerlocke in the image above. Facing the view (with the stairs to Hammerlocke behind you), hop on your bike, turn right, and pedal until you see a lake.
Cross this lake (which will often have large creatures) and go up the ramp until you see a series of stones. Jackpot! Each of these rock structures corresponds to a stone (Fire, Leaf, etc.), which are on the ground in bright view, just behind them.
There is another bonus too. Depending on the weather, a rare Pokémon may appear there and save you from using a stone or save time on a long evolutionary run. For example, if it's raining, you can find Aquali, or if it's dark, you can find Noctali. Good luck!
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