Final pre-season test lasts four days
Hydraulic leak ends McLaren's day with just seven laps completed
McLaren boss Ron Dennis expects Fernando Alonso to race in Australia
Williams' Felipe Massa sets fastest time of Barcelona tests
Hybrid issue forces Mercedes to end first day an hour early
First race is Australia on 15 March
But where, I hear you asking, is Lewis Hamilton while Massa is out there thundering around?
Well, he remains in the pits, his lap tally still standing at 48, as it was at lunchtime.
There's no suggestion of any major drama in the Merc garage.
He's been threatening it for a while now and finally he delivers. Felipe Massa sets the quickest time of the day with a 1:24.687, around two-tenths up on Lewis Hamilton's previous best.
Crucially, that lap from the Williams man came on the medium compound tyre that Hamilton was using this morning, and is just three-tenths off the best medium-tyre time of the two Barcelona tests so far, set by Hamilton's Mercedes team-mate Nico Rosberg.
Of course the usual rider about fuel levels applies, but that is nevertheless encouraging for everyone who does not want to see the Mercedes run and hide come Melbourne.
"While Felipe Massa is out there testing the new aero package on the Williams, his team-mate Valtteri Bottas is monitoring things from the pit wall."
Image source, KnutsonSainz's car has been cleared and we're under way again. Just over two and a half hours of this first day remains.
Carlos Sainz of Toro Rosso has stopped out on track and we have our second red flag of the day.
The Spanish rookie has stopped at turn 10 but there's no word on the nature of the problem yet.
Sainz has run well to this point, amassing 59 laps with a best time of 1:27.261.
Andrew Benson
Chief F1 writer
"Not long after Ron Dennis's half-hour news conference on the subject of Fernando Alonso's wellbeing, the team have suffered yet another blow.
"They say they are stopping running after discovering a water leak in the engine. Jenson Button had done just seven laps so far.
"If McLaren and Honda can't fix these repeated problems, its getting dangerously close to the point where even if Alonso is fit for Australia, you'd have to question whether there is any point him turning up."
Image source, Getty ImagesMeanwhile, back on track, Felipe Massa is continuing to pound round. He's now improved to a 1:25.133 - just a quarter of a second from Lewis Hamilton's best time.
McLaren on Twitter:, external "We're bringing the shutters down on today - a hydraulic leak means we need to change the engine. #TestingIsTesting"
McLaren's day ends after just seven laps, the best of which was more than six seconds shy of the benchmark pace of Mercedes' Lewis Hamilton. McLaren's troubled pre-season continues...
Lewis Hamilton is much loved at BBC Towers by those who cobble together our esteemed F1 Gossip page because the guy is social media gold.
When he's not posting a picture of himself sitting on the wing of his private jet, he's eyeballing Taylor Swift at the Brits - or, as below, taking a selfie in the cockpit of his Mercedes.
Keep up the good work Lewis. The Gossip column needs you.
Image source, Lewis Hamilton/TwitterAfter a brief lull, it's all go on track now. Felipe Massa has been busy since the resumption. He's now done more laps than anyone today - 52 as I type - with Williams seemingly switching to a race simulation for this afternoon.
Carlos Sainz is up to sixth place in the Toro Rosso thanks to a 1:27.640, while the Ferrari of Kimi Raikkonen has also improved. The Finn has done a 1:26.553 on the medium tyre and is up to fourth.
Image source, Getty ImagesRed Bull's Daniil Kvyat has provided the day's (minor) drama thus far, stalling his RB11 on his way out the pits to bring out the red flag inside 10 minutes.
Image source, Getty ImagesThis has been a rare sight today - a McLaren rolling out of the garage. Jenson Button has managed just seven laps so far as the team continue to try to get to grips with their new challenger.
Felipe Massa's early outing after lunch came close to his morning best - punching in a 1:25.963 - but with that he's back in the pits.
Ferrari's Kimi Raikkonen and Carlos Sainz of Toro Rosso also ventured out but also returned to the pit lane in quick order.
Still we wait for this day to catch light...
Thanks to all of you who played our blurry-picture game thing earlier (the title needs some work).
Among incorrect driver guesses were Ayrton Senna, Alain Prost, Allan McNish, Keke Rosberg and David Coulthard.
For the year, 1986, 1988, 1989, 1990, 1993 and 1996 were all wrong.
Erroneous track guesses included Adelaide, Silverstone, Estoril and Imola.
But plenty of you were right, and you told me it was Mika Hakkinen and his stricken McLaren, just moments after he had triggered a huge multi-car crash at the start of the 1994 German Grand Prix at Hockenheim.
Image source, Getty ImagesOk, lunch time over. Time for more track action.
The rain has stopped and Felipe Massa of Williams is the first man out.
Let's do this.
Time for more idle nonsense to pass the lunch break.
We have (very scientifically) blurred the below picture.
If you can work out from this who the driver and car is, what just happened and at which track and in which year it occurred, you will earn my lasting respect.
Answer using #bbcf1
Image source, Getty Images
Image source, EPAAndrew Benson and Lee McKenzie's story from today's McLaren news conference is now online.
McLaren chairman Ron Dennis said Fernando Alonso is "completely lucid", "normal" and should be fine to race in the season-opening Australian Grand Prix despite last Sunday's crash.
With the lunch break comes a chance to catch up on our caption competition we set you earlier. Here's a few of your responses to the below pic of Max and Jos Verstappen.
Nick Peart: "If you want to look old enough to drive in F1 I think we should get you a cap that fits... What about one of these?"
lfcrule1972: "Look Max, there's a new game on the App Store called FlappyMcLaren. Seems there are a lot of bugs in it though."
Richard Chappell "Look at this Max, Madonna was on stage and a Minotaur pulled her over by a cloak. Best Brit Awards ever don't you think?"
Chris Cullen: "So son, you just swipe right if you fancy her and left if you don't. Simple."
Image source, Getty ImagesThat's the morning session done. Here's the times and laps competed so far.
Mercedes' Lewis Hamilton leads the way in both lap time and mileage. At other end, the McLaren of Jenson Button is well off the pace after managing just seven laps.
1. Lewis Hamilton (Mercedes) 1:24.881 (48 laps)
2. Felipe Massa (Williams) 1:25.763 (29)
3. Daniil Kvyat (Red Bull) 1:25.947 (45)
4. Romain Grosjean (Lotus) 1:27.079 (33)
5. Marcus Ericsson (Sauber) 1:27.802 (33)
6. Kimi Raikkonen (Ferrari) 1:27.813 (28)
7. Carlos Sainz (Toro Rosso) 1:27.866 (34)
8. Jenson Button (McLaren) 1:31.479 (7)
Back on the asphalt, there's little to report.
That rain shower rather took the life out of what was already a low-key session. Lewis Hamilton and Daniil Kvyat went out in the Mercedes and Red Bull respectively, but their times were predictably well off the pace.
The lunch break looms.