WITH snow-laden hills behind her (if you squint hard enough through the gloom) this is a £1bn warship disappearing beneath the waves for the very first time.

New hunter-killer submarine HMS Astute has conducted her inaugural dive in open waters in the Scottish Exercise Areas (which are basically anywhere off the west coast…), chaperoned by HMS Montrose.

Astute has submerged before – conducting a ‘trim and basin dive’ in the deep basin at the BAE yard in Barrow-in-Furness.

Deep though that basin is, it cannot accommodate a fully-submerged 7,400-ton Astute class boat.

Luckily, the waters of western Scotland are more accommodating.

So with CO Cdr Andy Coles in charge, Astute left her home port of Faslane in mid-February for several weeks of trials (the boat’s inaugural passage from Barrow to the Clyde last November was entirely on the surface and less than pleasant; she endured Force 8 gales for a time).

Luckily, the gods were more forgiving this time around. Two days after departing Faslane, the boat passed that major milestone by completing her first, rather gentle dive.

We cannot, of course, tell you how deep she dived during these first test runs. But we can tell you that (a) it wasn’t especially deep (as faster and deeper dive trials are planned) and (b) she came back to the surface again.

All the while the trials were observed and monitored by HMS Montrose… whose job it is to hunt and, er, kill hunter-killers (enemy ones rather than our own, of course).

The Type 23 frigate is herself undergoing some thorough tests and trials after emerging from a multi-million-pound refit in Rosyth last autumn.