1.
If a business or project goes under, it becomes unable to continue in operation or in existence. (BUSINESS)
If one firm goes under it could provoke a cascade of bankruptcies.
= collapse
PHRASAL VERB: V P
2.
If a boat, ship, or person in a sea or river goes under, they sink below the surface of the water.
The ship went under, taking with her all her crew.
= sink
PHRASAL VERB: V P