In 16w20a, the new fallingdust particle has been added. This particle shows up underneath all blocks that have physics, if they have an air block underneath them (so if they would fall when updated).
These are all five blocks that have gravity:

• Sand
• Red Sand
• Gravel
• Anvils
• Dragon Eggs

That last one, the Dragon Egg, doesn't create the particle, though, when they should. This is seemingly a bug and should be fixed.