The Siberian Lake Some Geologists Think Was Made by the Tunguska Object

Lake Cheko is a small water body in central Siberia, oval rather than round, about 700 metres long, 360 metres wide, and 50 metres deep at its lowest point. It sits on the Kimchu River, about 8 kilometres northwest of the centre of the 1908 Tunguska blast — the region where roughly 80 million trees were flattened in a radial pattern on 30 June 1908 by the airburst explosion of an unidentified incoming object.

Cheko first appears on Soviet topographic maps in 1929. It does not appear on any prior Russian-Imperial map of the region. Whether this is because the lake did not exist before 1908 — or because the Imperial map coverage of that part of the Krasnoyarsk taiga was thin — has been the central question of a twenty-year scientific dispute.

The Italian theory

The Cheko-as-crater hypothesis is the work of a team based at the Marine Geology Institute of the Italian National Research Council in Bologna, led by the geologist Luca Gasperini. The team’s first Cheko expedition (1999) was originally part of a broader survey of the central Tunguska area; it returned with bathymetric soundings, sediment cores, and seismic data that the Bologna researchers thought were inconsistent with a normal Siberian glacial or thermokarst lake.

The published case appeared in Terra Nova in 2007 and in Geochemistry, Geophysics, Geosystems in 2009. The core arguments:

The lake is the wrong shape and depth for the regional geomorphology. Most lakes in the central Tunguska area are shallow circular depressions in the Permafrost. Cheko is deep, oval, and aligned along an axis that points back toward the 1908 blast epicentre.

The seismic data show a buried mound or anomalous body about 10 metres beneath the lake floor at the southern end, consistent with what the Bologna team interpreted as a buried impactor fragment of perhaps 1.5 metres diameter.

The sediment cores show no soft organic sediment older than approximately 1908. The pre-1908 layer is firm clay; the post-1908 layer is loose lacustrine silt. The carbon dating of the boundary is consistent (within the substantial error bars of carbon dating in this kind of disturbed sediment) with a 1908 formation date.

The trees around Cheko’s perimeter are noticeably younger than the trees a few kilometres away. The team interpreted this as evidence that the area around the lake had been cleared by the impact and the surviving forest had grown back since 1908.

The counter-case

The 2008 reply paper by Gareth Collins (Imperial College London) and colleagues argued the Bologna case was unconvincing on every count.

On the maps: pre-1908 Russian-Imperial topographic coverage of the Tunguska region was sparse but not nonexistent. A 1883 Russian map of the broader Yenisei basin shows water features in approximately the right area, which the Imperial College team read as plausible evidence that Cheko predated 1908. The Bologna team disputes the resolution and accuracy of the 1883 map.

On the geomorphology: small deep oval lakes do occur in permafrost terrain (the aufeis mechanism produces them through repeated cycles of ice-dam formation and erosion). The argument from shape and depth alone was not compelling.

On the buried anomaly: the seismic signal could be a buried tree stump, a basalt outcrop, or simply an artefact of the imaging technique. The Bologna team has not been able to drill it.

On the sediment cores: the soft-vs-firm boundary at approximately the 1908 level could equally represent any number of changes in the lake’s hydrology or vegetation around that period, not just an impact.

On the tree ages: the surrounding forest was systematically logged in the Soviet period; the apparent youth of the perimeter trees may have nothing to do with 1908.

The scientific majority position, as of the mid-2020s, is approximately that the Cheko hypothesis is interesting but unproven. The most direct test — drilling the buried seismic anomaly to determine whether it is a meteorite fragment — has not been attempted, partly because of the substantial cost of drilling through 50 metres of water and 10 metres of lake-floor sediment in remote Siberia, and partly because the Bologna team and its critics have moved on to other projects.

Why it matters

The Cheko question is not a small one. The 1908 Tunguska object remains the only large impactor of the modern era — the only event where humans observed (from a substantial but not enormous distance) the kind of city-killing collision that the inner Solar System produces approximately every century or two. The cosmochemical signature of the object — was it stony, was it cometary, was it metallic? — would be of immediate use to planetary defence research.

Without a physical sample, all the cosmochemical inferences come from the indirect proxies: ice-core anomalies, plant-ash traces, microscopic spherules in soil samples from the 1927 Kulik expedition area. These suggest a stony (chondritic) impactor of approximately 50–80 metres diameter; the energy release was approximately 10–15 megatons of TNT-equivalent. But the proxies are noisy and the sample sizes are small.

A confirmed crater containing actual fragments would resolve all of the open questions. If the Cheko theory is right, the southern-end seismic anomaly is sitting in 60 metres of cold dark Siberian water, waiting for someone to bring up a 1.5-metre piece of the 1908 object.

If the Cheko theory is wrong, the original 1927 Kulik observation still applies: the Tunguska object exploded above the ground and no significant solid fragment has ever been recovered.