Dow tumbles 500 points as oil's historic plunge worsens

· *US stocks slid further on Tuesday as oil continued to trade at negative levels.*
· *West Texas Intermediate contracts expiring in May traded as low as -$16.74 a barrel on Tuesday. **June contracts slipped as much as 42%, to $11.89 amid the turbulence.*
· *Stock futures erased some early gains after reports said North...