When the ground beneath you stops being solid, what do you do? Jump on it like a trampoline! Seriously, it may be a little dangerous but it looks like oodles of fun. Like you're walking on water only it's really still the ground.
The phenomenon is known as soil liquefaction and it gets pretty gnarly, as can be seen in the video below. It basically happens when the ground loses its rigidness and strength after an earthquake (or some other sort of change in stress condition) and causes it to move around like a liquid.
You can have a lot of fun with it! But the effects of loosey goosey ground in a city could cause a lot of damage. You might want to jump up and down on it, but you definitely don't want your house built on top of jelly ground.