Quicksilver Fountain 
Artifact
$3.65 #13681 0.74 23.0 years
At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.
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Cards similar to Quicksilver Fountain expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Harbinger of the Seas74%
$1.60↓#3933↑1.66↑2.3 years↓
"Nonbasic lands are Islands."
Xolatoyac, the Smiling Flood72%
$0.46↓#5126↑0.50↓2.9 years↓
"Whenever this card enters or attacks, put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it."
Eluge, the Shoreless Sea72%
$4.42↑#3149↑0.712.2 years↓
"Whenever this card enters or attacks, put a flood counter on target land. It's an Island in addition to its other types for as long as it has a flood counter on it."
+1 more matching line
Conversion72%
$0.26↓#26641↓1.16↑33.2 years↑
"All Mountains are Plains."
Glaciers70%
$0.54↓#29298↓1.25↑31.4 years↑
"All Mountains are Plains."
Zhao, the Moon Slayer70%
$0.28↓#10040↑1.10↑0.9 years↓
"As long as this card has a conqueror counter on him, nonbasic lands are Mountains."