2 options

1) subfloor rotted doe to leak DANGER DANGER

2) cheap fiberglass tub or the concrete poured support has given way and the tub is not being properly supported, DANGER DANGER

Both can cause you to fall through the floor and also cause thousands of dollars of water damage

Thank you, all. It's definitely a lightweight material (Acrylic or FRP), not cast-iron. The house was built in the '80s, no idea when the tub was installed.

We haven't taken baths before this spring, and neither have our guests.

This is fairly common for an acrylic tub. likely means that it was not properly installed...could also be the result of a bathfitter type installation where the new tub was not properly adhered to the old. And yes, the water you put in weighs a lot, 8.3 lbs per gallon or something if my high school physics works. So 25 gallons is 207 lbs.

Fiberglass can do this. A cast-iron one wouldn't obviously, so I bet it's the material the tub is made from.

If you have never taken a bath there before, has anyone else? It's possible it has always been like this, depending on what material the tub is out of, and how the concrete was poured underneath.

It's something we noted during construction and had them adjust, but it was not something dangerous.

Hmmm....what material is the tub made out of? My guess it's the cheap plastic "bathfitter" crap.

Bet the sub-floor is rotted. Call the Landlord ASAP and consider showering at the gym until he/she figures out the problem. (yes, a bathtub full of water weighs a lot.)

I am not sure but I would not get in that bath again, or put any water in it til you alert your landlord and get a structural engineer out there PRONTO

Bathtub started to cave/ flex mildly under a standing person's weight (imagine the sensation of standing on a plastic tub that looks flat, but flexes under your feet).

We've been renting this place for 3 years, but I only recently started taking baths (until 2 months ago we showered exclusively). DH and I are of normal weight. Could this be because of the additional weight of the bathtub water? Please post if you've ever experienced this.