Dwight D. Eisenhower:

The last three years of Eisenhower's second term in office were ones of relatively good health. Eventually after leaving the White House, he suffered several additional and ultimately crippling heart attacks. A severe heart attack in August 1965 largely ended his participation in public affairs. In August 1966 he began to show symptoms of cholecystitis, for which he underwent surgery on December 12, 1966, when his gallbladder was removed, containing 16 gallstones. After Eisenhower's death in 1969 (see below), an autopsy unexpectedly revealed an adrenal pheochromocytoma, a benign adrenaline-secreting tumor that may have made the President more vulnerable to heart disease. Eisenhower suffered seven heart attacks in total from 1955 until his death.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What did Eisenhower have removed via surgery on December 12, 1966?
gallbladder