This is why I don't add a feeding tube.

Yes of course a remote controlled firmly sealing motorized door on the other end of the tube would solve this problem, but I know of no such kit and building such a thing from scratch is beyond my abilities. Besides which, food is not the limiting factor timewise, I can fit enough food in there to last for months. The limiting factor is how long I can leave it down before the litter needs changing, and if I need to bring it up once a month anyway I may as well use that opportunity to resupply food. Ergo, no feeding tube.

Airlocks and submarines

So, I have been prototyping different ideas for a safe airlock so that I can resupply each habitat with food without bringing it to the surface. It's a harder problem than it seems like. A moon pool won't suffice, since it lets in too much humidity over time and the hamsters quickly learn they can dive out of it. A double door airlock won't work because obviously they aren't smart enough to open the inner door by themselves and I can figure out no way of doing so from outside.

So, what I have come up with is a single door airlock/moon pool combo. When the door is closed, air coming down through the tunnel slowly forces water out the one-way valve in the floor of the airlock chamber until it is empty. When it is open, it floods the chamber only up to the top of the little doorway cut into the climbing tunnel, air continues to bubble out. I have researched relatively waterproof foods that come in large chunks that I can quickly put into the airlock, and found that glazed "treat sticks" fit the bill; Nuts, seeds and other goodies glued together with honey. Having submerged these and then allowed them to dry, they do not become soggy except after many submersions (the sugary outer layer eventually dissolves.) I think I could easily stick one of these into the airlock, close the door, purge the water, and it would be very edible after a minute or two.

I have also been thinking about Hamsub Mk2. The Mk1 was slow, clumsy, cramped and limited to shallow water. I would like the Mk2. to be fast, maneuverable, spacious and to carry it's own air supply. Towards that end, I was looking at the state of the art in RC subs.  As you can see, these days RC subs can be extremely fast, maneuverable and sophisticated, featuring everything from individually controllable pivoting gun turrets to battery propelled torpedos that really fire.

I have also discovered that the Playmobil toy line includes a number of surprisingly sophisticated toy submersibles with sealable, strongly watertight cabins, electric propulsion, and functioning ballast tanks. They also make a smaller but equally watertight "diving bell" that could be stripped down to the watertight portion and then added to a high end RC sub as the cockpit.

The specific sub I had my eye on for the basis of Hamsub Mk2. can be seen here. It's very robust and already ballasted appropriately such that adding the modified toy diving bell to the front would not affect the balance (in place of the viewing bubble, which does not appear to be water tight.) For the onboard air supply, I would sling one of these micro scuba air tanks under the sub's body. I'd then affix a valve that would let me set it to very slowly release the contents into the 'crew cabin', which would vent bubbles continuously out a one way valve in the floor. Because both the air canister and the proposed 'crew cabin' come pre-ballasted to neutral buoyancy, neither would upset the balance of the sub itself. Based on the rate at which a single hamster consumes air, that canister could easily provide over an hour of life support between refills. As it's still an ambient pressure design, the safe depth would be no deeper than 21 feet, although that's ample considering the city itself will be no deeper than 8 feet.

P.S. big thanks to Scroton, who recently donated a 50 watt solar panel and additional battery pack; With these, it is possible to keep a colony running perpetually even far from any source of power, by charging one battery while the other is in use, and periodically swapping them. The solar panel will ensure a single battery lasts around two days on a charge and the air pump has it's own integrated battery so it will keep running during the swap.

Hamlab IV

Structure 2 of 4 that will comprise the first actual multi-structure 'city', Hamlab IV (better name pending) has two individual sleeping quarters and a large shared atrium with transparent dome cieling and an interior water supply (for a later experiment in refilling the water bottle while the habitat is submerged.) Pardon the water droplets, it was filled with plastic shavings after I finished the outer shell and I've just washed it out. Below I've included the concept illustration, a photo of the work in progress habitat alone, next to the completed Hamlab II for comparison, and then a side shot to show the view through the sleeping pod's window. In keeping with the new design philosophy, each occupant has a private space to themselves, and the overall size/weight does not make it difficult to transport/deploy. However I did choose the "multiple linked modules" because of a commenter's observation that single-structure habitats aren't as fun to watch because there aren't any tunnels for the hamsternauts to go through. I'll try to include such tunnels and multiple modules on the next habitat as well. Together with the Mk. IV, Hamlab II and whatever the fourth building is, the completed city will house nine hamsters in total. That can be increased to thirteen with the addition of another air pump and two more habitats, as the battery pack has three outlets in total, but the initial city will have four buildings.

HAMLAB 1 and 2

Here's Hamlab 2, the two story observation tower built from a 100 CDR spindle and a 20 CDR spindle plus fishing weights and assorted other parts. I'm pleased with the unusual design. You can see the much smaller Hamlab 1 next to it, unoccupied. It was only suitable for 5 or 10 minute uses due to the small interior room and lack of a water bottle. Hamlab 2 comfortably houses two hamsters and includes a food dish, water bottle, nesting fluff and silica litter to prevent the buildup of humidity or waste fumes. If this approach of building smaller individual habitats proves more practical than the gigantic Mk. III all in one approach I may simply build a city out of unconnected individual living structures, on account of pet guides advising not to put hamsters from different sibling groups together. With this more gradual, affordable approach I think a city of perhaps eight structures total is feasible at a depth of between 3 and 7 feet.

The Mark VI.

Another completely infeasible fantasy, due to the cost. I had the idea while looking for companies that sell transparent plastic domes in various sizes. The largest I found was 8 feet in diameter and something like $36,000. Presumably that's due to the low manufacturing volume, as people don't generally have a widespread, burning need for 8 foot diameter transparent plastic domes. However, were my funding somehow unlimited, this would be my ultimate masterpiece/cry for help:

Floating halfway between the surface and the ocean floor, anchored by steel cables in water 500 feet deep, the Mark VI would be positioned on the periphery of the gulf stream so as to sap energy from the currents via it's pivoting turbine, but not be subject to significant stress on the cables. The panels around the bottom of the shaft are passive silicone membrane CO2 exchangers; They refresh the air inside without using any electricity. In fact, the first experiment demonstrating this phenomenon was done using...an underwater hamster habitat! :O

As you can see, the one wall of the enclosure made up of the super thin silicone membrane isn't terribly large compared to the animal it's providing oxygen for. I simply decided to overdo it a little so that this hovering "seastation" could accommodate larger numbers, and go for longer periods between maintenance. Of course the plants help too, but it doesn't contain nearly enough to do the job without external support. The enclosure itself is 1 atmosphere (due to the depth, which insulates it against storms) contains a closed loop biosphere to provide the hamsters with food (plants, insects, seeds) decompose their waste, and by way of one of those tacky little multi-tier faux rock indoor fountains, a source of fresh water (passively desalinated by equipment in the lower half of the enclosure) that does not sit stagnant and allow insects to breed therein. Suspended from the top of the fountain is the artificial sun; A heater bulb, as used in incubators, provides heat and light that simulates natural sunlight exposure. Around the base of the fountain you may notice openings for various webcams that transmit live footage of the habitat interior over the internet. Finally, there's a docking hub just above the turbine section that permits an autonomous model sub, purpose built for this station, to dock to it and deposit additional food and chews should they be necessary, as well as to add additional hamsters (the sub carries a small compressed air supply sufficient for the round trip and is refilled after each use) and possibly to transfer 'crew' between multiple stations. If I ever win the lottery, expect an ocean full of these. Alternatively, if anyone from Pixar or Dreamworks is reading this by chance, it seems like all of this would fit neatly into one of their movies. Animals doing things they don't normally do when watched by humans, their hidden world cobbled together from junk, it's Secret of NiMH underwater. Call me, let's make this happen.

The Mark V

.....If I had money to burn, that is. (Estimated cost: $1050) Why yes, that is a floating surface station with vertical shaft down to a one atmosphere observatory. Why do you ask?

Progress on the Mark 4

The habitat with ballast containers and finalized layout:

A demo of the LED light fixture with the small USB battery I will be using for backup in case of temporary power failure (the air pump also has battery backup, but 14 hours of it. The LED lights will run for maybe 4 or 5 hours on battery. Both backups recharge for next time once mains power is restored)