CREATE TABLE table_19624 ("Explosion" CLOB, "Location" CLOB, "Date" CLOB, "Yield  ( approximate ) " CLOB, "Altitude  ( km ) " CLOB, "Nation of Origin" CLOB)

What explosion had an altitude of 539 km?
SELECT "Explosion" FROM table_19624 WHERE "Altitude (km)" = '539'
SELECT "Explosion" FROM "table_19624" WHERE "Altitude (km)" = '539'