There's a lot of space debris in space, orbitting the Earth.  When those orbits decay, as they do over time, the debris burns up upon re-entering the Earth's atmosphere.  

No biggie.

Unless the debris is really big to begin with — like an out-of-use satellite.  Some of the debris survives re-entry and crashes to Earth.  We've seen a few of those recently: An old NASA 6-ton atmospheric research satellite came tumbling down in September, and a 3-ton German science satellite followed suit in October.  The surviving parts of those old satellites ended up in the oceans.

But there's another killer out there: the Russian Phobos-Ground satellite.  It's way bigger than those other two satellites — it's 14.6 tons.

Also, it's not an old satellite.  In fact, it wasn't supposed to be a satellite at all.   It was launched in November 2011, and a glitch left it stranded in orbit around Earth instead of bound for Mars to collect soil samples.

And now it is coming back down to Earth, carrying within it about 12 tons of highly toxic fuel that was supposed to take it to Mars.  Some think that the fuel is probably frozen, and it will become UNfrozen during re-entry, and then spread in tiny droplets over some area — perhaps even a populated area.

The Russian space agency Roscosmos' latest forecast has the unmanned Phobos-Ground probe falling out of Earth's orbit Sunday or Monday, with the median time placing it over the Indian Ocean just north of Madagascar.  Of course, these is the same agency which built and launched the piece-of-crap probe, so who knows where it will come crashing down. 

London and New York are along its flight path.

More info here.