After accumulating data-sort-value="25000">25,000g, Demetrius will visit the Farm and inform the player that he would like to use the cave to set up research specimens. He'll give the option to choose between fruit bats (which leave fruit in the cave) or mushrooms. This choice is permanent and cannot be reversed.
Fruit bats
Fruit bats leave various fruit in the Cave on your farm. This is a great option if you are large into making Artisan Goods as it gives you a slow but steady supply of fruit even out of season. This also helps speed up finishing the Artisan Bundle in the first year or two before you have enough money to buy all the Fruit Trees necessary for the bundles. Fruit Tree fruit make excellent gifts for villagers because they are universal likes (with the lone exception of Pierre and apples), readily accessible early game, do not require a kitchen, and cheaper than many other universal likes. This seems to be the best late game use outside of making Artisan Goods, Shipping them, finishing the Artisan Bundle, or eating them for extra energy even though they don't replenish very much health.
Mushrooms
Six plots will be added to the cave where certain types of mushrooms will randomly grow. Five different types will appear, including all four that are required to craft the Life Elixir. Even though Common Mushrooms, Morels, and Chanterelles, are the ones that most commonly appear, this is still the best way to craft the elixir as often as possible. This makes the Mushroom option a great one for those who love to go into The Mines, especially since most mushrooms can be hard to find otherwise. This is also a good choice for players that have grown, or plan to grow, fruit trees and do not need the extra fruit provided by the fruit bats. The mushrooms regrow every day at a random time between 4:00PM and 10:00PM if they were harvested the day prior. If they are not harvested on the day they grew back, they will stay overnight and can be harvested the following day, however even if the player harvests them in the morning, they will not grow back until the next day at the aforementioned time.