The Farm Cave is an underground location accessed through a passage near the northern edge of the Farm. Initially it is just a small empty cave. After you accumulate 
25,000g in total earnings, Demetrius will visit the Farm in the morning and inform you that he would like to use the cave to set up research specimens. He'll give you 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
Interior of the Fruit Bat cave
Fruit bats leave various fruit in the Cave on your farm overnight. This is a great option for making Artisan Goods as it gives you a slow but steady supply of fruit, particularly foraged berries, even out of their respective seasons. It also helps complete the 
Artisan Bundle in the Pantry in the first year or two, before you have enough money to buy all the necessary Fruit Trees.
Fruit Tree fruits make excellent gifts for villagers because they are universal likes, readily accessible early in the game, and do not require a kitchen. Gifting may be the best late-game use for fruit, other than making Artisan Goods, Shipping them, finishing the Bundles, or eating them for extra energy (even though they don't replenish very much health).
Every night, there is a chance of new fruit spawning. If the cave is empty, there is a 48% chance of no new fruit; 25% chance of 1 fruit; 13% chance of 2 fruit; 7% chance of 3 fruit; 7% chance of 4 or more fruit.[1] The average spawn rate is 1.06 fruit per night. There is no hard limit on the maximum number of fruit in one night, other than the available floor space (50 tiles).
Fruit spawned in the cave always remains there until the player collects it (the cave contents are never automatically reset, even at the end of the week or the end of the month). Uncollected fruit reduces the space available for new fruit to spawn. For example, ten uncollected fruit increases the chances of no new fruit from 48% to 54%. Placing equipment or craftable flooring in the cave also reduces the space available for fruit.
Fruit found in the cave can be of any quality (normal, silver, or gold). If the Botanist Profession is chosen, the fruit will be iridium quality. The Gatherer profession gives the player a chance of obtaining two fruit. Each fruit obtained from the cave gives the player 7 Foraging experience points.
Mushrooms
Six boxes 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.
Cave mushroom production takes one to two days. Unlike other processing, the time when mushrooms are ready for harvest depends upon the time when the player goes to sleep: processing is faster if the player goes to sleep later.[2] The earliest mushrooms can be ready is 12:40pm, if the player goes to sleep at 2am. If the player goes to sleep at or before 6pm, mushrooms will not be ready until two days later, resulting in a missed day of production. These processing times apply both to newly-placed mushroom boxes and to existing ones that were just harvested.
Mushrooms grown in the cave are always of normal quality, even if the Botanist Profession is chosen. Harvesting mushrooms from the Cave does not award the player with any Foraging experience points, and the Gatherer Profession does not award double mushrooms.
References
- ↑ 1.0 1.1 Fruit spawning is controlled in the game code by FarmCave::DayUpdate. The code loops repeatedly, with a 34% chance of the loop terminating on each iteration. Otherwise, the spawn location is randomly selected from one of 90 tiles. Fruit only spawns if the selected tile is a valid location. 40 of the tiles are never valid (in the cave walls); existing fruit, equipment, or flooring in the cave will also make tiles invalid.
- ↑ The mushroom processing times are controlled by the game code in Object::DayUpdate case 128. For any empty mushroom box, the processing time is set to 3000 - Game1.timeOfDay minutes. Because this code is triggered when the player goes to sleep, Game1.timeOfDay ends up being the time when the player goes to sleep (e.g., going to sleep at 1:30 am (2530) means processing takes 3000-2530=470 minutes). However, no processing happens during the first night (the code handles all overnight-processing updates before calling Object::DayUpdate), meaning the processing start time is 6am the next morning.
- ↑ See Object::DayUpdate in the game code.
Wine
Jelly
Cranberry Candy
Summer Seeds
Fruit Salad
Fall Seeds
Plum Pudding
Blackberry Cobbler
Fried Mushroom
Tom Kha Soup
Stir Fry
Life Elixir
