(~s)
1.
Quicksand is deep, wet sand that you sink into if you try to walk on it.
The sandbank was uncertain, like ~ under his feet.
N-UNCOUNT: also N in pl
2.
You can refer to a situation as ~ when you want to suggest that it is dangerous or difficult to escape from, or does not provide a strong basis for what you are doing.
The research seemed founded on ~.
N-UNCOUNT: also N in pl