Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What explosion had an altitude of 539 km?

### Inputs:
table_name: table, columns: ['Explosion', 'Location', 'Date', 'Yield (approximate)', 'Altitude (km)', 'Nation of Origin']

### Response:
SELECT Explosion FROM table WHERE Altitude (km) = 539