Where is the Alex Rider short story underground?

The Alex Rider short story "Underground" is part of the "Stormbreaker" anthology. This anthology includes several short stories set in the Alex Rider universe, including "Secret Weapon" and "The Killing Zone".

The anthology was published in 2006, after the release of the "Stormbreaker" novel.

It's important to note that "Underground" is not a full-length novel but a short story within this collection. You can find it in the "Stormbreaker" anthology.