Miramar Mystery

I must have been past this old tunnel/bunker a thousand times and never noticed it. Set into the hillside by Burnham Wharf, I haven't found out exactly what it is but agree it's likely to be from WW2 and is a military magazine, used to store supplies/ammo?

It's obviously been recently frequented by humans judging by the tagging and empty beer bottles and other rubbish. It was pitch black inside and despite being 6'2" and 100kg, I was so scared something was "coming to get me", I was taking photos with the flash almost on continuous mode!!!