if a serious situation deepens, it gets worse - used especially in news reports:
The recession continues to deepen.
a deepening international crisis
to become stronger or greater, or to make something stronger or greater:
become stronger[intransitive and transitive]
Jeanne liked Simon as a friend but she did not want the relationship to deepen.
The idea only deepened his gloom.
The mystery deepened (=became even more mysterious).
Students explore new ideas as they deepen their understanding (=understand more) of the subject.
if someone's smile or frown deepens, they smile even more or frown even more:
expression on somebody's face[intransitive] literary
Her worried frown deepened.
if water deepens, or if someone deepens it, it becomes deeper:
water[intransitive and transitive]
The river deepens beyond the town.
The harbour was deepened to take bigger boats.
if light or a colour deepens, it becomes darker:
The twilight deepened.
if a sound deepens, it becomes lower:
His voice deepened as he relaxed.
if your breathing deepens, you take more air into your lungs