thanks for your amazing reply!
i think sometimes english is not my first language. unfortunately, i don’t really have a second either. 
blush yeah, i only remembered the right word later.
in fact i have a basement and the water supply pipes are exposed and logically placed. after reading your response, i went back down and stared at things. i think that the person who remodeled was a diy handyman type ( happy mutant? ) and – like myself – not always quite sure what they were doing.
the drain pipe, running from where i think the old sink was, terminates just below the kitchen floor. in the basement, i can see that the the exposed floor joists extend just a tad lower than that. basically, i think they didn’t have a pipe cutter – so rather than cut the drain to below the joists, drop a drain down from the new sink into the basement, and run it sideways under the floor, to connect from there – they instead wormed it through the stud, and connected the new sink to the old sink’s drain behind the cabinetry.
im guessing something in there rusted out or broke a seal. i’m leery of cutting through more of my walls and cabinets – there’s some electrical wiring in there somewhere – and i don’t have the means to cut the pipe in the basement myself. my current plan is to wait till next paycheck, and call a licensed plumber. but, i may call around to some friends and see what tools they have in the meantime.