Sinking Sands is the training ground to practice your Tempos, attacks and any Hosts that you may have unlocked. Once damaged, the Dummies in the map will instantly regenerate any lost health, which means you can damage it as much as you want (though there are a few ways to kill it, as seen in the Training Dummy's page.)
If you don't walk off the edge for 10 minutes, you will automatically be brought to the upgrade menu of Peacock Peaks.
After unlocking the tempos 
Cell and 
Tear, an invincible version of Resident Mind will appear on the southern border. Walking towards him will make you teleport to the Endless Timeless.
Appearance[]
Sinking Sands is a blue, sandy platform. The skybox appears to be an inverted image of a desert landscape.