CREATE TABLE table_19620 ("Explosion" TEXT, "Location" TEXT, "Date" TEXT, "Yield  ( approximate ) " TEXT, "Altitude  ( km ) " TEXT, "Nation of Origin" TEXT)

What was the altitude of the explosion Hardtack Teak?
SELECT "Altitude (km)" FROM table_19620 WHERE "Explosion" = 'Hardtack Teak'
SELECT "Altitude (km)" FROM "table_19620" WHERE "Explosion" = 'Hardtack Teak'