Aleppo is the largest city in Syria. There is thousands of years of history in it, thousands of years of architecture, which foreign invaders took just a few years to destroy.
Apparently, thanks to the western intelligence that provides the rebels with guidance, the rebels learned that the government troops were in the Carlton Hotel in Aleppo. They were provided with a map of underground tunnels previously used as mines and equipment needed to drill their way immediately under the hotel. Then they rigged the spot with explosives and remotely set it off, blowing the hotel up with a huge explosion.
First video shows the massive explosion the tunnel bomb caused:
Second video shows the underground prep work and a different angle of the explosion: