user: ### NOW ANSWER:
Question: What explosion had an altitude of 539 km?
Columns: ['Explosion', 'Location', 'Date', 'Yield (approximate)', 'Altitude (km)', 'Nation of Origin']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Hardtack Teak', 'Johnston Island ( Pacific )', '1958-08-01', '3.8 megatons', '76.8', 'United States']
SQL:

assistant: SELECT "Explosion" FROM "Table" WHERE "Altitude (km)" = '539';