[RichH]

Airex core???
Probably a simple ''void'' or gas pocket that formed during layup - a fairly common occurance in older boats. I''d inject polyester resin to fill the void. I''d avoid epoxy for this injection in case the void is too deep causing excess heat build up with an epoxy that may break the surface with extreme pressure as the epoxy ''kicks''.

 

[RichH]

Repair:.... make a tiny hole to inject the mixed resin with a large plastic syringe. If you have access to a paramedic or nurse or physician.. get a very large bore hypodermic needle to attach to the syringe. Make a very teeny hole to allow air venting while you inject. Mix up the resin and push it in with the loaded syringe. Keep the injection holes as absolutely as small as you can... less surface repair.
Tape over the whole area with masking tape before you inject so you dont ''spill'' resin on the surface.
Airex is ''plastic foam'' used for core material... You could also have balsa core.... Airex seems to bond less well than balsa.
Good luck.