I have NO idea how balanced this is. Probably not very.

It's reminiscent of Thawing Glaciers, which was crazily powerful. This is rather more restricted given the restriction to mountains, but still extremely good in any slow-ish deck. It'd be more balanced if like the Glaciers it put the land card in your hand.

Ah yes, I kinda missed that this wouldn't use up your land drop, a bit tooo accelerant. Changing it.

Did you mean for this to be Basic?

I kinda wanted them to be able to search out more of themselves, but no, I'm not certain yet. Would it be too horrible if it was a mountain? Can you have a non-basic mountain?

There are several non-basic lands with basic land types (as in this search) notably the Ravnica shock lands, which does exactly what you want. (It can be search for with "search for a mountain" or occasionally "search for a land card with a basic land type" but not "search for a basic land")

Note that anything with type "mountain" can automatically tap for red mana (it's not printed on basic mountains, the rules explicitly say that anything with that subtype are treated as if it had the "T: add to your mana pool") which you may or may not want here.

"Basic land types" are defined in the rules to be the five you expect.

"Basic lands" are exactly those lands with "basic" in the supertype, and the rules say you can have any number in a deck. Currently that's the five basic lands, plus the five basic snow-covered lands. In principle wizards can make basic lands with more rules text (I tried it in Gentleman Magicians) but they never have and probably never will.

And I think "search for a mountain" means "search for a card with mountain subtype" not "search for a card named mountain", but I'm not sure.

So it would certainly be permitted for this to be a "Land -- Mountain", and I like the flavour, but it would definitely need to keep "comes into play tapped" since it can tap for or search. Alternatively it could not be a mountain but explicitly say "search for a card named mountain or redmine".

I'm not sure what would balance it best. I think the current version is good but not completely broken, but I'm not sure exactly how good.

It'd be capitalised as "Search your library for a Mountain card", as on Arid Mesa. Arid Mesa can find nonbasic Mountains such as Steam Vents or Madblind Mountain. By contrast, Jund Panorama explicitly says "basic Mountain card". (To complete the assortment, Rampant Growth and friends say "basic land card" which means a card with type Land and supertype Basic; and Perilous Forays was a nasty hack to find any of the Ravnica duals as well as just normal basic lands.)

It'd be very odd to give this type Basic; I wouldn't recommend it. Making it a Mountain itself is a fun possibility - it does increase the power level significantly over just letting it tap for , due to the aforementioned searchers.

Ok, it's probably best if it isn't a mountain.

I was mainly trying to design a "Don't manascrew me!" land.

I like the ability to search itself itself out - it gives you a nice choice between land now or later - but as you say, just the ability to strip your deck of land is probably good enough. Although... let it search for a Mountain or Mine? Let's try that, see how it looks.

And yes, I'd intend this to be able to get Strip Mine, Urza's Mine etc. I... probably shouldn't allow it to get Howling Mine though. What to people think?

At the moment, Urza's Mine is the only land with the printed type Mine:

It'd be fine to errata Strip Mine to give it the land type Mine if you wanted. You couldn't give Howling Mine or Magma Mine that type without a lot more errata, because Mine is a type that can only go on lands as things stand at the moment.

By the way, the search template needs ", reveal it" and "then shuffle your library".

Oh.. sucrose. If it can search for mines then it can mana fix by searching out the other coloured mines. Which I didn't want. Bah humbug.

... would I still run "tap to get a mountain"? Sometimes. So it's probably strong enough that way.

I drafted this fairly highly, because I wanted to play it. I had 2 of this, 3 Deepest Delvings, and a (CL05), in a red-black deck - which meant I only had 6 mountains, and often ran out pretty quickly. At which point this becomes useless (and fodder for saccing to Dwarven Miner or (CL05) to get back my Tunnel Skeleton).

So far this seems as I'd suspected: this is good, perfectly playable, but nowhere near overpowered. It's certainly no Thawing Glaciers. It might be a bit good in formats with Steam Vents and Taiga, admittedly.

Um, what's bl05 meant to be?

Yeah, it was good - but not overpoweredly scarily so. Hoorah!

Bah. Your basic lands don't have basic land codes. (CL05).

They've flipped form being common to basic and back far too many times for me to keep track of. Will fix. But how on earth did you know the 05 part without knowing the other bit?

Because I'd wanted to link that card in a comment somewhere else, and I forget its name, so I looked up its code then to enter in a link of the form (((-CL05))). Then I misremembered the code now.