Not as artistic as I would have liked perhaps, but I am having trouble with my laptop again.
In any case, it provides useful visual information about the Rock Configurations in the first three Chambers of the Mine as well as information about the Underground Lake.
Although this video really was created to show the Rock Configurations in the first three Chambers, a brief explanation of Mining may be in order.
First of all, you need to unlock Mining in general by working Part-Time for Rudolph five times before you will be able to unlock Mining for yourself. In order to experience some of the Story Quest Events that are linked to the Mine, you must satisfy requirements in terms of the total number of items that you have obtained from the Mine. Mineral Crystals do not count where these totals are concerned, as you can 'create' them yourself using the Transformer 1 Machine. In order to experience all possible Events in the Story Quests linked to the Mine, you need to have obtained at least 30 items from the Mine. This is a fairly low requirement, as are any of the Requirements in HoLV. It actually represents the bare minimum any player should be able to obtain if he/she visits the Mine a few times.
A new Chamber should be unlocked at the beginning of any new Season, BUT it is possible that there are minimum requirements that must be met here as well. Obviously, if you did not unlock mining yet, the Chambers will remain barred with fallen masonry. There may be some requirements in terms of the total number of items that players have obtained previously in the Mine as well. These totals would be fairly minimal, but Mineral Crystals would not be included in the count. The reason why Mineral Crystals are not counted is because you can create them yourself by placing Grass in the Transformer 1 Machine in the same manner in which you can create Special Fish Bait by placing Lumber in the Transformer 2 Machine.
If you have unlocked no new Chambers in the Mine NOR the Underground Lake, then I would assume it is a mining requirement rather than an additional fishing requirement.
In my game and those of players who were able to access the Underground Lake as soon as Ponta spoke of it, we had been mining on a regular basis since the start of the game, gaining access to the Mine as early as we could in Spring by working 5 times for Rudolph. We had been selling items from the Mine regularly to local merchants and had given gifts to Rudolph to raise his FP. We experienced two of his Friendship Events before Autumn... Rudolph's total FP is the only other variable that comes to mind.
Incidentally, the only Harvest Moon and Rune Factory games that I cover are the genuine authentic versions published by Natsume and XSeed. True fans of Harvest Moon and Rune Factory will obtain the genuine article, even if it involves some personal sacrifice, in the interest of helping to support the people who make these games available to us. If you are using bootlegs or illegal downloads, then you are NOT supporting the creators of these fabulous games. Illegal downloads and bootlegs tend to have all sorts of 'bugs' and problems, from an inability to save your game properly to corrupted data or options that do not work.