The Plan
I've decided to do a small set of cavern floor plans to add to my Dungeon set. These will allow me to have 'natural' caves for my non fantasy/dungeon RPGs such as my Pirates game.
The caverns set doesn't need to be as extensive as the dungeon set so I'm only going to make the bare minimum to begin with. There's always the possibility that inspiration will strike as I go along and so I may end up adding special pieces later.
I plan to make a single large cave roughly 300mm x 300mm, another large cave but split into two with a straight edge so the two parts can merge into dungeon rooms if necessary or just be used as a second big cave, two 300mm x 150mm caves and four 150mm x 150mm small caves.
These will be connected by a set of eight 150mm x 60mm tunnels/passages.
I'll print a small set of stalagmites and cave entrances to add variety. These will primarily be from here and here.
​I already have plenty of rocks and a river that can be combined with the floor plans if needed.

The Build
There's only one straight cut for this set so I'm hoping to be able to cut almost everything with shears. Be warned, I started with scissors and got some nasty blisters. Shears are better.

This is the single big cavern, the easiest of the set to cut.

For the rest I marked the reverse of the cork tile with guide lines.
After I cut out the irregular shape for the second big cave I used the guide line to cut it in two with a knife and ruler.

Two 300mm x 150mm caves.

Four 150mm x 150mm caves.

The eight 150mm x 60mm passages. These can be chained like sausages to make any sort of winding path I may require.

And that completed the basics. Next, the fun part.

Painting
I'm going to paint the caves in my sand colours like the dungeon rooms. 

Once everything had been given a coat of the base colour I added some texture with my basing compound (base paint, PVA and sand) and some slate fragments. The basing compound holds the slate pieces firmly in place.

Then I dry brushed with the two highlight colours.

A set of stalagmites. Eight hours and thirty-six minutes to print, total cost £1.55.

Three large cave entrances. Seven hours and thirty-two minutes to print with a cost of £1.80.

I also found these rather nice natural rock pillars here.
Six hours and forty-four minutes to print a set of four costing £1.90 in total.