Authorities also have been battling thick ice that has formed on the Danube in Serbia, blocking all river traffic and endangering rafts and boats parked by the river banks. Thick fog has caused numerous flight delays and cancellations at London Heathrow and other area airports. The Met Office forecasting service said visibility at Heathrow had been reduced to roughly 100 meters (110 yards), leading to an overall slowdown in operations. Rescue crews are considering whether to start using heavy equipment to speed up the search for 23 people still buried under the ruins of a central Italy hotel crushed by an avalanche. Firefighter spokesman Luca Cari said emergency crews were working with an "operational hypothesis" that the tons of snow that plowed into the Hotel Rigopiano on Jan.