The Country of Fog, also known as The Country of Mist, is Chapter 23 of Tsubasa: Reservoir Chronicles.
The fog is growing thicker, and Kurogane and Fai are still walking around. They've been walking for quite some time now, but there is still no sign of life. Not even a villager. Kurogane remarks that it is scary, and Fye smiles, telling him that he's here. Kurogane replies chirpily, "Kurogane is very happy!" Turns out that voice wasn't really Kurogane, but Mokona impersonating him. Mokona and Fai joke around, and Kurogane marches off angrily. They approach the lake and stop in surprise. Syaoran tells Sakura to hide. She falls to the ground and enters another flashback. Her father, Clow, is talking to Fujitaka. Fujitaka introduces Sakura to his son, who is about the same age as her. Clow tells her that the young boy suffered a painful life in the past, but he doesn't know it. He began to change his cold and distant personality little by little after meeting Fujitaka. What the child needs is warmth, and Clow hopes that his little daughter will give him that. "A smile that can rival the sunlight of springtime." Present Sakura opens her eyes, wondering who that boy could have been. As Syaoran dives down deeper and deeper into the lake, he also remembers a flashback. He had just returned from the princess's birthday party, and he felt very warm inside after she smiled at him despite not knowing about his past.
Syaoran nears the bottom of the lake, and spies a city there. It's a very small city though. A big fish swims over him, and he realizes that the fish is the sun of the underwater city. The fish drops a scale down, and Syaoran catches it. He smiles as he remembers one of Fujitaka's old sayings. "You will find the happiness that belongs only to you. Although you may not know what it is right now, you will surely find it someday." Syaoran swims back to the surface. As he takes off his goggles, a worried Mokona runs towards him. "Sakura is...!"