4) THE DAY THE SKY FELL IN: ASTEROID IMPACT AND THE FALL OF ROME
...
"But the Rome of that era came close to suffering a far worse calamity. A
small metallic asteroid descended from the sky, making a hypervelocity
impact in an Apennine valley just 60 miles east of the city. This
bus-sized lump of cosmic detritus vaporised as it hit the ground. In
doing so, it released energy equivalent to around 200 kilotonnes of TNT:
around 15 times the power of the atomic bomb that levelled Hiroshima in
1945."
...
http://abob.libs.uga.edu/bobk/ccc/cc020803.html
Also the 'AND FINALLY: WHY CYNICS FIND COMFORT IN "COSMIC ANNIHILATION" ' is an amusing read.