Why is ETH ultra sound money?

I'm hearing quite a lot about ETH being touted as "ultra sound money." Could you please elaborate on this? What exactly makes ETH qualify as ultra sound money? I'm curious to know about the specific characteristics or attributes that contribute to this classification. Is it its scarcity? Its programmability? Its security? Or perhaps a combination of all these factors? Also, how does ETH compare to other cryptocurrencies in terms of being sound money? Could you provide some insights into why ETH stands out in this regard? I'm eager to learn more about this concept and how it applies to the world of cryptocurrency.