[annie123 (OP)]

Divers report plastic water bottles on the sea bottom several km down.Why don't they stay floating on the top?I can't make them sink to the bottom of a water filled garbage bin.

[evan_au]

It depends on the type of plastic; air-filled polystyrene foam is unlikely to sink.

Most plastics are manufactured from hydrocarbons, and oil floats on water, so you could guess that many hydrocarbon plastics will float on water.

But some plastics also contain Fluorine (eg Teflon) or Chlorine (eg Poly VinylChloride: PVC), and this will increase the density beyond a pure hydrocarbon. Some of these plastics are much more dense than water (water=1 g/cm³, or slightly higher for sea water).
See: http://www.dotmar.com.au/density.html

Note that some plastics are slightly porous, and water may penetrate the plastic over weeks, slowly increasing the density until it sinks.

[chris]

Also, plastics pushed down by waves and currents will feel a greater pressure which may compress any embodied air making the relative density of the material increase, so it sinks, like a submarine.

[PmbPhy]

Divers report plastic water bottles on the sea bottom several km down.Why don't they stay floating on the top?I can't make them sink to the bottom of a water filled garbage bin.

I believe that you're confusing density with mass. While a piece of plastic may appear to be light it has a density which is large enough so that it will sink. Perhaps you're confusing mass with density. Consider how a WW-II battleship can stay afloat even though it has a huge amount of mass. With this in mind think about what would happen in you had a solid cube of plastic and you put it in water. It would sink.

[annie123 (OP)]

to Pmb
But what about a plastic bottle which divers say end up  so far down? WHy can't I make I sink in a bin? Even when it's filled with water?

[Colin2B]

There are a number of reasons.
The ocean isn't pure clean water, it obtains a large amount of suspended matter particularly in coastal areas. A plastic bottle or bag near the shore will soon gain a small amount of sand and it doesn't take much to sink a plastic bottle.
Other types of floating plastic which don't have a cavity can also sink because over time they are colonised by small sea creatures e.g. barnacles or just algae, which increases their weight.
As Chris says, some plastics contain very small amounts of air within their structure and are surprisingly porous. Once these are pushed below the surface the water pressure compresses these micro air pockets and the buoyancy reduces.

[ygengineering]

I think it may be the material of the plastic bag, or it may be soaked in water, or it may be that the garbage bag has other garbage,

[vhfpmr]

The majority of plastics are denser than water:
https://plasticranger.com/density-of-plastics/

[Zer0]

Some folks are Real Diggers!
They might fare well in Minecraft.
🤭

P.S. - Take measures Drastic...
Cut the Plastic..
Become Fantastic.
🥳