This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
You can't do anything here unless you have charged the Pendant up in
12,000 BC so you can break the seal here.
Inside are two treasure chests, but you can only take from one. The one
on the left is the Safe Helm; the one on the right is the Swallow sword.
Personally, the FAQ maintainer recommends the Safe Helm, but you can
take whatever you like.