Aquarist

The Cave is part of the Oceanarium. It's located in the back-left as you enter, past the cafe. The cave has 5 sections, each with a fairly simple tank to fill. The achievement will unlock once you have completed them all and cleared the final set of planks.

This will be an end-game achievement, as it will take plenty of time to earn your way through the preceding levels to get here and to unlock access.

I've included a brief summary and approximate prices for each section. I did this after getting Rich Man so I had money to burn, but it may be useful if you want to save-scum this achievement while still working towards the aforementioned one. I also did this after completing the story, but you could do this as soon as you unlock the Oceanarium if you have the cash.

First, you need to buy access into the caves. There are two entrances. I bought the first one, closest to entrance, for $20,000. I'm not sure you can start at the other one. The caves form a loop. There are numerous wooden planks blocking access. I won't mention all of them but use the hammer to clear them. Remember to press for a list of requirements.

If I left the Oceanarium or finished for the day, the objective changed at the top of the screen. I was still able to continue with the task but if you don't know where you're up to it may be a problem. Best to do this all in one go; it should only take 15 minutes or so.

• In the first section, you need to fill a Shrimp tank in the wall ~ $2000
• Access reward and clear path to next section
• Glow-in-the-dark fish tank ~ $4000
• Turn off lights. There are 2 switches on the wall outside, near the bin. I didn't see any difference but I was able to proceed. If it doesn't seem to update, try going to another location and coming back again.
• Collect reward
• Electric Catfish tank ~ $1500
• Collect reward
• Jelly Fish tank ~ $1500
• Collect reward
• Arapaima tank ~$4000
• Destroy final planks
• With the bare basics, this added just over $1100 income per day

So, overall it cost around $33,000 but this is a very approximate number. I only kept basic track of what I was spending.