1 Lambert: A solarium? What on earth would dwarves, a race that typically spends its entire life underground, want with a solarium?
2 Dwalin: Dwarves nidd vitamin D, too, ye ken! {translation: Dwarves need vitamin D too, you know!}
3 Lambert: But surely you can get that from food you can easily grow underground. From... uh... what are the natural food sources of vitamin D?
4 Dwalin: Ocean fish...

Vitamin D is an important organic chemical in maintaining levels of calcium and phosphorus in the bloodstream and in bone structure, as well as regulating production of parathyroid hormone and stimulating the immune system. Deficiency in vitamin D leads to the disease known as rickets in children and osteomalacia in adults, both characterised by loss of bone density and vulnerability to joint pain and fractures.

Dwarves are not known for suffering from joint pains and bone fractures - indeed, if anything they are renowned for being sturdy and strong. So clearly they are getting a more than adequate supply of vitamin D.

The problem is the major source of vitamin D is exposure to sunlight. Our skin generates sufficient vitamin D for us to be healthy with a short daily dose of sun exposure. And if there's another thing dwarves are not known for, it's sunbathing.

You can also get vitamin D from your food. Unfortunately, very few foods contain significant amounts of vitamin D. In our modern age, several staple foods such as milk, margarine, and breakfast cereals are artificially fortified with vitamin D. But for naturally occurring vitamin D, you pretty much have to go to one place: the ocean. Fatty fish such as tuna, salmon, sardines, and eels are about it.

It's hard raising tuna in an underground mine.

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2017-09-23 Rerun commentary: Because after all: You can tune a piano, but you can't tuna mine.

Also... notice the cat chasing the rat across the panels here? Hmmm...