Because we upgraded from PHP 5 to PHP 7 during the last sync, a change occurred that may affect whether or not a mineral deposit exists on terrain squares that did not already have an existing deposit prospected.

Between version 5 and version 7, PHP changed how seeded random number generation works. Because we use a seeded random number to determine whether you succeeded in finding a deposit (which is what makes it so you can't just constantly prospect a square and eventually succeed), that means that any squares which do not yet have a deposit on them now have a different "success chance" number than they did before. This means that prospecting those squares again may yield a deposit where there was none before. This affects any terrain square which does not currently have a deposit on it, so terrain squares with existing deposits already prospected will be unchanged.