antennae bolted to underground grate on eastern pk greenway?

this is just west of the franklin avenue stop on the eastern parkway greenway (the north side one). there's also a big rock on top of the grate.

anyone know its purpose(s)?

my gal and i were walking the dog on saturday and she saw a well-dressed man (suit; sans uniform) pick up the rock, open the grate and hoist himself down along with his duffle bag and close the grate before disappearing underground. maybe it was lex luthor? i missed the whole thing as I was picking up our dog's number two. of course.

anyway, my gal notified the authorities, reasoning it was better to tell someone because she "saw something" as the ad goes, and to her, it spiked a bit from the generally continuous level of "normal weird new york" experienced regardless.

we differed on our opinions; i thought it was possibly some art project/performance piece, or just some dude trying to buck subway fare. maybe he lives down there and has rabbit-ear analog tv hookup? and maybe that hookup is dysfunctional now due to the digital transition (dunno if old analog tv would work down there anyway)? anyway, what's up with the antennae, et al?

any knowledge, guesstimates, opinions of this bit of ephemera?

thanks,