On April 16, after elaborate reconnaissance, the Union's fleet steamed into position below the forts and prepared for the Battle of Forts Jackson and St. Philip. On April 18, the mortar boats opened fire. Their shells fell with great accuracy, and although one of the boats was sunk by counter-fire and two more were disabled, Fort Jackson was seriously damaged. However, the defenses were by no means crippled, even after a second bombardment on April 19. A formidable obstacle to the advance of the Union main fleet was a boom between the forts, designed to detain vessels under close fire if they attempted to run past. Gunboats were repeatedly sent at night to endeavor to destroy the barrier, but they had little success. US Navy bombardment of the forts continued, disabling only a few Confederate guns. The gunners of Fort Jackson were under cover and limited in their ability to respond.