Scroll down a bit. M.2 on the backside.

Listed under Storage in the specs.

Isn't M.2 storage specific?

eg not a useful PCIe slot for anything other than plugging in an SSD

M.2 has full four lanes PCIe, you can use M.2 to PCIe x4 adapter and use all the standard PCIe cards.

M.2 is a connector specification it has nothing to do with speed. M.2 supports applications such as WIFI, USB, SATA, and PCIe. M.2 SSDs are faster and store more data than most mSATA cards. M.2 SSDs support PCIe 3.0, SATA 3.0 and USB 3.0 interfaces, while mSATA only supports SATA. M.2 SATA SSDs have similar performance to mSATA cards, but M.2 PCIe cards are faster. SATA SSDs have a maximum speed of 600 MB per second, while M.2 PCIe cards can hit 4 GB per second.

PCIe support also allows M.2 cards to take advantage of the nonvolatile memory express (NVMe) protocol, which brings a large performance advantage over other types of interfaces due to reduced latency, increased IOPS and lower power consumption.

Thanks, hadn't heard of them before. This looks like the right kind of thing:

https://www.newegg.com/p/17Z-00AP-00018?Description=m.2%20ad...