Warnings from the oil tanker industry that too few of the ships are being built are haunting the market after Houthi attacks on shipping caused widespread diversions in petroleum trades.

Oil rose toward $64 a barrel, a year to the day after futures for the U.S. benchmark collapsed below zero, with the world’s most important commodity extending its rally on bets for better demand.

Oil fell from its strongest close in nine months, hampered by weaker risk sentiment in global markets.

Oil slumped below $12 a barrel as the biggest oil ETF said it will sell out of its June WTI futures position, adding to the downward pressure from a huge glut.

As crude prices continue to get dragged down by a global glut that shows no signs of abating, oil traders may find some profit by going to sea.