1.
The ~ features of an object, event, or situation are not obvious, and it may be difficult to discover or reveal them.
To stop a problem you have to understand its ~ causes...
I think that the ~ problem is education, unemployment and bad housing.
ADJ: ADJ n
2.
You describe something as ~ when it is below the surface of something else.
...hills with the hard ~ rock poking through the turf...
ADJ: ADJ n
3.
see also underlie