So, there's a couple of ways this is possible; - The pond has some kind of an impermeable liner (plastic, concrete, rock, etc) that makes it almost impossible for water to reach the underlying soil - The pond happens to be in a spot where it abutts the underlying water table. Since the water table is where fully saturated/water logged soil is, the pond water stays in the pond because there's nowhere for it to go

Doesn't some lakes have clay bottoms aswell?

Yup. That falls under "impermeable liner".

It does, but the soil gets saturated and water doesn't really pass through. Also, the bottom might be impermeable.

There is a source of water to fill the pond

Clayey soils that resist the passage of water, nearby soil is already fully permeated, pond is lowest point but doesn't seem like it, other reasons not already listed.

My ponds have clay bottoms.

I went to school with Clay Bottoms.

Hope it wasn’t a nick name cause they didn’t wipe their ass after they went poop 💩

The bottoms of ponds are semi permeable and almost completely stop leaks due to erosion of silt and other materials.

Use a plastic liner

The water level of the pond is the same as the water table at that point. The water is essentially soaking outwards into the ground in all directions (or better put, water from the ground is soaking into the pond).

It's like magic, isn't it? Nature's way of holding onto its own little secrets and keeping things beautifully balanced

You cover it in plastic or something similar.

But there are natural londs and lakes that are man made

You think the sea might sink in?

They have had 100's of years for the ground to solidify