sealed underground structure near the windmills?

07/08/26 - posted by Cole

Hi all, hoping the community's sharp eyes can help identify something.

A friend told me about an underground structure somewhere in the western end of Golden Gate Park, near the windmills (Dutch/Murphy). From the description: it's accessed by two angled bulkhead-style doors flush with the ground (like old root-cellar or coal-chute doors), currently locked shut, with a set of steps leading down once opened.
I haven't been able to track down what this is or was used for. My best guesses so far are:

An old cistern/reservoir access tied to the windmill water-pumping system (the Dutch Windmill fed a 200,000-gallon reservoir when it was built in 1903)
A cellar tied to one of the millwright's cottages
Some kind of WWII-era coastal defense or utility structure?

Does anyone recognize this, know its history, or have old photos/maps showing it? Even a rough sense of which decade it dates from would help. Happy to share more details or a location pin if that's useful.

Thanks!