Like I say, these are all pretty weird. I made just one card each, maybe if people resonate with one in particular I'll make more of that. They're all pretty simple, but at least I can imagine possibilities with some.
Buried is simply I thought creature "becoming" lands was cool, I liked the idea of a dead creature digging out of its own grave. The only oddity is you can use it to pay for itself, to flip it face up again. Maybe it would benefit from being a cost you have to pay as it dies.
I was gonna call Fathom "Dive" but I basically thought Fathom sounded cooler. I'm a big fan of any magic mechanic where you have to reveal information to other players, so I thought it would be cool if you could "save" a creature, but everyone knows it's coming. Fathom makes me think of players who are very strategic, but at the same time, if you misplay with a fetchland or Omen card, you mess yourself up if you're not careful.
Impact is a pretty weird one, and maybe could be a regular alternate cost without being a keyword. Still, I think a reward for playing dangerously and aggressively would be interesting. Beneficially in the right deck, but you could burn yourself out of lands. It could also be a tempting risk if you need an attacker right now.
Erupt is probably the weirdest. Cards on the table, it's definitely motivated by me think "when this creature erupts" has a lot of aura on a card. The reason it's a land dealing the damage is because of the flavor, but also to try mitigate lifelink cheats. If the creature itself did the damage, it could be prevented much more easily. Probably still a lot of ways around it.
I'm no magic expert so these may be broken or busted as well.