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The Tunguska Event, sometimes called the Tunguska explosion, was a massive explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m.[1] (0:14 UT, 7:02 a.m. local solar time[2]) on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time).[2]

The explosion was most likely caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 miles) above Earth's surface. Different studies have yielded varying estimates for the object's size, with general agreement that it was a few tens of metres across.[3]

Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons[4] to as high as 30 megatons[5] of TNT, with 10–15 megatons the most likely[5] - about 1000 times as powerful as the bomb dropped on Hiroshima, Japan and about one third the power of Tsar Bomba.[6] The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). The earthquake from the blast measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area.[7] This possibility has helped to spark discussion of asteroid deflection strategies.

The Tunguska event is believed to be the largest impact event on land in Earth's recent history;[8] impacts of similar size in remote ocean areas would have gone unnoticed before the advent of global satellite monitoring in the 1960s and 1970s.

The Tunguska event is the strongest, but not the only, significant meterorite airburst in recent history. A selection of similar events follows. This list is quite biased, since recording of meteorite explosive yields is relatively recent:
Date Place Yield of explosion (TNT equivalent) Height of explosion Remarks
June 30, 1908 60 kilometres westnorthwest of Vanavara, at 60°55' N, 101°57'E in Krasnoyarsk Krai, Imperial Russia 10–15 Mt 8.5 km Tunguska event
August 13, 1930 Curuçá River Area, Amazonas, Brazil 0.1-1.0 Mt
May 31, 1965 Southeast Canada 600 t 13 km 1g material from meteorite found
September 17, 1966 Lake Huron, Michigan, United States 600 t 13 km No material from meteorite found
February 5, 1967 Vilna Alberta, Canada 600 t 13 km Two very small fragments found - 48 mg and 94 mg.

Stored at University of Alberta, in Edmonton. [30]
September 22, 1979 Southern Indian Ocean 2 kt
January 19, 1993 Lugo, Northern Italy > 10 kt > -20a
September 25, 2002 Bodaybo, Russia 0.5 – 5 kt
June 6, 2002 Mediterranean Sea between Libya and Greece 26 kt

A Tunguska event would only be a problem if it occurs over a metropolitan area.

Pretty well everything I've read states they never found evidenc eof a meteorite or fragments of it that would have been the cause of such an explosion.
Other theories state it may have been a bit of anti-matter.
Some think a pin-sized "blackhole" may have temporairly opened.
We may never know exactly what did cause such destruction.
As the cosmic theory goes..."sh*t happens".

E.m. Kolesnikov1, Corresponding Author Contact Information, T. Boettger2 and N.V. Kolesnikova1

1Geological Faculty of Moscow State University, 119899 Moscow, Russia

2Research Group of Palaeoclimatology, Institute of Geophysics and Geology, University of Leipzig, D-04303 Leipzig, Germany

Revised 24 November 1998;
accepted 6 December 1998.
Available online 7 June 1999.

Abstract

Method of a search for traces of Tunguska Cosmic Body (TCB) material using layer-by-layer analysis of the isotopic composition of light elements in peat has been offered. Four peat columns sampled at the explosion epicentre indicated significant carbon and hydrogen isotopic effects in its near catastrophic layers. The shifts, opposite in direction, for carbon (?13C reaches +4.3‰) and hydrogen (?D reaches -22‰) cannot be attributed to any known terrestrial reasons (fall-out of terrestrial dust and fire soot; emission from the Earth of oil–gas streams; climate changes, humification of peat, and so on). Moreover, the isotopic effects are clearly associated with the area and with the time of the 1908 event. They are absent in the uppermost and the lowest peat layers and also in the control peat columns sampled at the remote places. Since calculated d13C value for an admixture of carbon (+51–64‰) is very high, these effects may not be explained by contamination of peat with material similar to ordinary chondrites or achondrites, too. Such heavy carbon occurs in the most primitive CI and CM types of carbonaceous chondrites. However, C/Ir ratio in a cosmic admixture is 10,000 times as many as in CI chondrites that points to cometary nature of the TCB. The isotopic effects are in agreement with the increase of the Ir content observed in peat, but, at the same time, small content of Ir points to the low content of dust in the Tunguska comet that sharply differs it from Halleys comet.