Why did one american soldier describe the japanese as not on iwo jima but in it?

Tunneling and caves--the Japanese soldiers were known to create tunnels and caves in the mountains of Iwo Jima.

The tunnels within the shear shoreline cliffs made taking the island difficult. As American soldiers attempted to scale the cliffs they were met with Japanese popping out of the tunnels. Eventually US soldiers began using flame throwers to clear out tunnels to advance.