The Wandering Lake is a traveling island that constantly shifts location.
Anywhere in the world, really. The Wandering Lake is an island not connected to the bottom of the sea, instead floating on the surface. It's location ranges from any of the Four Seas to even the Grand Line. However, it only shifts from one side of the Red Line to the other once every one thousand years, though how remains unexplained. It is theorized that it somehow passes up Reverse Mountain, but no one near Reverse Mountain has ever seen this happen.
The Wandering Lake is a natural land formation, consisting of a rocky island with a large lake and a small beach complete with a palm tree. It is unknown what made this island begin to travel, what keeps it afloat, or what keeps it going. It is theorized that air pockets in the rocks might keep it afloat. It is also theorized that the lake was once on a cliff of a tall island, but broke off either due to the weight of the water, or perhaps due to the island breaking apart and being destroyed by some natural disaster. But all of these are only theories, and none of them can be proven.
It is generally unknown what purpose, if any, it served in the ancient worlds. We know little about the Wandering Lake.
The island serves as a rather lucky find in today's world. It's quite a rare chance to come across the Wandering Lake when sailing, as it never goes near any major islands. However, it is almost impossible to land on it, as the island is very rocky aside from the small beach, and never stops moving. Only certain individuals have the abilities to land on the island. These individuals have commented that the beach is extremely soft, and the lake water is so clear that you can see fish swimming in it.
It's current location is unknown to all but the fish living in the island's waters and the birds flying in the skies above it.