What was the terrain in the Battle of Thermopylae

A pass with the sea on one side and mountains on the other. Beyond the mountains was a trafficable route which the Persians eventuually woke up to.

Blocking the pass made the Persians try to bypass it by sea, which was the reason for the blocking – to force a sea battle to try to eliminate the amphibious threat of the Persians to the southern Greek cities. The sea battle failed, and the pass was abandoned os its purpose had gone.