The hole in the theory
On June 30, 1908, an object exploded in the atmosphere over Siberia with the force of 10-15 megatons of TNT. The Tunguska event, as it became known, flattened approximately 2,150 square kilometers of taiga forest, felling an estimated 80 million trees. The scientific consensus holds that a stony asteroid or comet, perhaps 50 to 60 meters wide, detonated at an altitude of 5 to 10 kilometers. This airburst explains the widespread destruction without a primary impact crater, a mystery of the event.
About 8 kilometers north-northwest of the explosion's hypocenter lies Lake Cheko, a small bowl-shaped body of water. For decades, this lake was another feature on the map. That changed in 2007, when a team of Italian scientists led by Luca Gasperini proposed a compelling hypothesis: Lake Cheko is the impact crater from a fragment of the Tunguska object that survived the main aerial explosion.
Their evidence centers on the lake's unusual morphology. It has a distinct funnel shape, reaching a depth of about 50 meters, unlike other more gently sloped thermokarst lakes in the region. Acoustic soundings of the lake floor support this unique conical shape. The Italian team's seismic and magnetic surveys identified a density anomaly 10 meters below the lakebed at its deepest point, which they interpreted as a possible buried fragment of the impactor.
Sediments of time
The debate intensifies below the lakebed. Gasperini's team extracted a 175-centimeter-long sediment core from the lake's center. They discovered that the top meter of sediment consists of normal, finely layered lacustrine deposits. Below this, the material abruptly changes to a coarser, chaotic jumble. Using lead-210 and cesium-137 dating, they concluded the transition occurred around the time of the Tunguska event, suggesting the lake is only about a century old. Pollen analysis supports this, showing abundant aquatic plant remains only in the upper, post-1908 sediment layers.
The counter-argument is substantial. A Russian investigation in 1961 had already dismissed the idea, estimating the lake's age at over 5,000 years based on its thick silt deposits. More recently, in 2017, another team of Russian scientists presented contradictory sediment analysis. Their research determined an age of at least 280 years for the lake, using radionuclide markers from mid-20th century nuclear testing to calculate a consistent sedimentation rate of 3.6 to 4.6 millimeters per year. Other researchers point to the existence of mature trees, far older than a century, still standing around the lake's perimeter, which should have been obliterated by a crater-forming impact. The discovery of other nearby lakes with similar funnel-like shapes further suggests that Cheko's morphology may not be unique, but rather a result of regional geology, such as karst processes or melting permafrost.