Burrowing

One red mana to make a creature unblockable, conditional on the defending player controlling a Mountain: that conditional is the entire design, and it tells you everything about the metagame Alpha was built for. Wizards once treated basic land types as a meaningful axis of combat math rather than just a mana source, and the landwalk cycle assumed every game would feature a manabase whose colors told you something useful about whether your evasion would fire. The wrinkle is that the land-type axis never actually disappeared; it just stopped being something players bothered to read. Any permanent with the Mountain subtype switches mountainwalk on, so dual lands, shock lands, and triomes all count, which means the more colors an opponent splashes, the more incidentally they hand you the evasion: a modern multicolor manabase makes this kind of evasion more reliable, not less. The catch is the commitment. An Aura announces itself before combat, so you have to spend the red mana before you know whether the defending player will keep a Mountain in play; a manabase with the wrong land types simply never switches the evasion on and leaves you holding a one-mana enchantment doing nothing, no corpse required. Flying shipped in Alpha alongside landwalk and always offered cleaner, condition-free evasion. Burrowing belongs to the same lost school as the original landwalk creatures: cards tuned to a world where reading the opposing manabase was a real skill, rather than one where the land types are everywhere but nobody checks.