I installed Mint 19 on this 112 GB SSD last week, replacing Windows.
I believe I partitioned it with

512MB EFI
16 GB swap
the rest I used for /
and put /home, /tmp and /var on an HDD

Working well all week, so I looked at disk in Gparted just to check before I do the same to another Windows box.

That 16 GB swap has just disappeared.

Does anybody know what's up here, or have I just gone to la-la-land?

My guess is something happened, who knows what, during the installation. But . . . . First, a swap partition is not really necessary. It only comes into play if memory is full or close to full.
The swap partition is normally the size of memory so the system can swap all out if necessary. Do you really have 16GB of memory? I use a 2GB swap partition with 8GB of memory and to the best of my knowledge it has never been used. If you really feel you need a swap partition of some size, go back into gparted and create one. If it disappears then you'll know something nefarious is really afoot and it might be worth tracking down.

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P.S. No, I don't think you have gone to la-la land but then remember that computers are NOT inanimate objects. They actually have souls, are evil, and are out to take over the world. Doing a pretty good job of it, I must say.

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Thanks.

I did figure out that I don't need a swap partition - I'd forgotten about "swapfile", which the system is using on its own. Cool, eh?

I'm just curious, and like you say, computers are evil. This is obvious when you think "panpsychism" - the entire universe is alive. And is under the aegis of The Imp of the Perverse (Murphy's true identity)

As a side note - you know there really was a Murphy, don't you? I know there are a lot of urban legends, but I was told the story by one who said he knew Murphy at China Lake Naval Air Station.

Yeah, heard that.
What's curious is how a local story at an American air-base could have spread so far and so fast in the 1940's, or maybe the '50s, such that by the '60s it was ubiquitous.
Who needed the Internet?

It would be interesting to know if Gparted showed 16GB of unallocated space instead of the 16GB swap as originally set up...
with your drive situation it may not be a concern, but for myself, I found a swap partition better for my situation. I am only working with 2GB RAM and a swapfile was resident in the / filesystem. When I changed to a swap partition rather that a swapfile I found better overall performance, generally lass lag time, plus the swapfile size is now available to the disk. Just food for thought...

No, the space for the swap partition disappeared, so now I think I maybe just plain forked up.

Just for curiosity, what is the output of fdisk -l?

As an aside, the machine I'm using right now how 16 GB RAM and an 8GB /swap. With that much RAM, it barely touches the swap. Right now, with a little over of two days of uptime and VM running, it still has not used any swap since the reboot.

For that disk:
What does that tell you?

What it tells ME is that there's an unused part of the disk, from sector 250068992 to sector 250069680, which is only 689 sectors, not big enough to have been your swap.
So probably the creation of the EFI partition overwrote your previous swap partition.

Thanks.
Looks like it's ok - working fine.