What's over 26 miles long and contains more people in lycra than almost the whole of the 1980s? It can only be this year's London Marathon, taking place on Sunday 28 April 2019.
With thousands of people pelting across the city and main roads closed from 7am, it's going to be a tight squeeze if you're planning to watch or cheer someone on.
But we've got a few handy tips too if you want to avoid the Tower Bridge crush this Sunday.
Track your London Marathon runner
Obviously you could use something like the free Find My Friends app. But from 7am on Sunday, there'll be a live tracking page on the London Marathon website, where you can keep tabs on your runner using their race number or surname. This is a less stalker-like option if you've come to cheer on a colleague.
There's even an official app for 2019, available to download for both iPhone and Android. This bad boy lets you track your pals during the race, plus other important info such as first aid stations and, y'know, the nearest pub.
We don't really need to tell you this one, but we'll just remind you what Britain is like at this time of year: variable. Keep those shoes flat and stash your stuff in a small bag — no one likes a huge rucksack-wielding space-hogger. Bring your waterproof, sun cream and plenty of water.
Plan transport around the London Marathon
Many main roads in central London will be closed from 7am, with buses on diversion or terminating early from 6.30am-7.30pm. Some Santander Cycles docking stations will be closed. London Underground, Overground and DLR is generally the best way to get around, with extra services running throughout the day. Be aware it's likely to get very busy though; worse than the rush hour crush on the Central line.
The MBNA Thames Clippers will be running special extra services, which can take you from central London to Greenwich Pier, close to the start of the race. Runners even get 1/3 off tickets on marathon day.
Avoid these London Marathon viewing spots
THE FINISH: It's a tough call as it's obviously one of the most exciting places, but trust us, it's not worth the chaos. It tends to get hectic anywhere from mile 24 onwards.
THE CUTTY SARK: It's a beautiful backdrop and you get all the buzz from being close to the start of the race, but the pay-off is humungous crowds. Greenwich town centre in general is a bit of a bun-fight, and transport to/from this area is particularly busy.
TOWER BRIDGE: What's not to love, watching the race from a London landmark? Plenty, that's what. Spectators are really keen on this one, so they start queuing from almost the middle of the night. Have 40 more winks and watch from somewhere else with more space and less early-rising required.
MILE MARKERS: 'Meet you at mile 17', 1,000 or so people have already said to their runners before you. It's going to be really hard to spot whoever you're cheering on in a sea of vests with numbers on. Unless they're in fancy dress, in which case — we'll give you this one.
2019 CLOSURES: Due to building works, a couple of popular viewing spots are restricted this year. Near mile 15, large scale work continues around the Heron Quay roundabout so don't plan to head there as it'll be fenced off. Similar story for Marsh Wall (between Isle of Dogs and Canary Wharf) around the 18 mile mark, where sections of the footway are currently closed.
Places to go to watch the London Marathon
THE START: This is a good call. It can be even more exciting than the finish line, with runners lining up, bursting with energy. As there are different start areas (have a look here to see their location), it's not as congested. You won't be able to accompany your runner right to the start line as it's race entrants only, but you can still soak up the atmosphere. The mass race starts from 10.10am this year, but you'll need to get down much earlier if you want a good spot.
ISLE OF DOGS: The loop south of Canary Wharf tends to be the least crowded of the course. Miles 14-21 are generally good bets and are served by a number of DLR stations close to the route. Keep in mind the 2019 closures mentioned above, though.
MUDCHUTE: It's less packed with people here, and it's easily accessible by DLR plus not too far from Canary Wharf. There's also a handy city farm and large park near by, if you're getting cramped by the crowds.
Of course there's always watching it on TV. BBC One shows coverage from 10am, so you won't miss a thing. It's also on BBC Radio 5 Live and BBC Radio London.
What to do after the London Marathon
Runners with charities normally have somewhere reserved to go and recover, which is perfect. But what do the others do? There's a post-race meet and greet area around Horse Guards Parade, but you can expect this to be packed. The London Marathon website estimates that it'll take around half an hour for a runner to reach this area once they've passed the finish line.
A better use of your time might be to head straight to a pub, bar, restaurant or coffee shop for a celebratory beverage, once your limping runner has crossed the finish.
Venues within a mile of the finishing line will be chocka, so if you haven't reserved anywhere already, the best strategy is to choose a pub/bar/whatever right now and book a table. Then — as you're not going to the finish line — head to the place by noon and keep a couple of seats for you and your runner, so they can make their way there without worrying where you are. You'll be following them on the tracking system, so you won't lose each other. Mobile signal around the end of the race can be patchy, so make sure you've communicated your plans in advance.
It's also going to be a struggle getting home, so let the crush die down as much as you can before helping your blistered runner towards public transport. Remember to check TfL on the day for the latest alterations and status updates.