U.S. oil futures sank below the key $40-per-barrel level, and now crude could test August lows and possibly fall into the low $30s per barrel.

The latest U.S. government data show that oil supplies continue to build, though at a slower pace in the past week than in the prior week. But the figures reinforced that the global oil supply glut — with some 3 billion barrels stockpiled worldwide — is showing no signs of abating.