How Powerful Was Gamera Heisei? Calculating the Energy of the Legion Explosion

Legion’s detonation in Gamera 2: Attack of Legion remains one of the most devastating energy releases ever portrayed in kaiju cinema. The explosion doesn’t just engulf the battlefield—it carves a massive crater into the earth and nearly vaporizes Gamera in the process. By examining the shape of that crater, the density of the surrounding terrain, and even the scaling laws used for real nuclear fireballs, we can estimate the true magnitude of the blast Gamera survived. In this analysis, we break down the crater geometry, calculate the displaced mass, and compare multiple yield-estimation methods to determine just how powerful Legion’s explosion really was.

Using Gamera’s shell as a reference point, we can estimate the scale of the massive crater left after the Legion explosion. By comparing its visible dimensions, we can derive approximate measurements in meters.

To simplify calculations, the values have been rounded up or down depending on which number they were closest to. Based on this, we have the following data:

Crater diameter: 942 meters
Radius 1: 471 meters

Crater depth: 66 meters

Smaller crater diameter: 553 meters

Radius 2: 272 meters

The shape of the crater most closely resembles a truncated cone — not a perfect representation of reality, but accurate enough to provide a solid estimation for our analysis.

Using the shape of a sphere or a paraboloid would yield larger results due to the irregular form of the crater. Another possible approach would be to use a 3D model, but unfortunately, I wasn’t able to find one that matched the actual structure closely enough to produce a precise volume.

Therefore, for this estimation, we’ll rely on the truncated cone formula, which provides a reasonable approximation:

$V = \frac{1}{3} \pi h (R_1^2 + R_2^2 + R_1 R_2)$

Where:

h = crater depth
R₁ = upper crater radius (rim)

R₂ = lower crater radius (base)

By substituting the values:

$V = \frac{1}{3} \pi (66) (471^2 + 272^2 + 471×272) = 29,300,422 \, m^3$

For density, I’ll use the average value of Sendai rock, approximately 2700 kg/m³.

$2700 × 29,300,422 = 79,111,139,468 kg$

That means Gamera endured an explosion powerful enough to displace nearly 79 billion kilograms of rock.

To bracket the explosion energy, we can use an empirical analogue: the Sedan crater (a 1962 cratering test). That test used a yield of 435 terajoules (≈104 kilotons of TNT) and produced a crater that displaced roughly 11 million tonnes of material (some of which was vaporized, pulverized or fragmented). From those numbers you can derive a rough specific energy required to make the crater:

$\text{Specific energy (Sedan)} \approx \frac{4.35\times10^{14}\ \text{J}}{1.1\times10^{10}\ \text{kg}} \approx 3.955\times10^{4}\ \text{J/kg}$

(≈ 39,554 J/kg — the value you used.)

Applying that per-kg cratering energy to the mass we computed for Gamera’s crater:

$E \approx 3.955\times10^{4}\ \frac{\text{J}}{\text{kg}} \times 7.9111\times10^{10}\ \text{kg} \approx 3.12\times10^{15}\ \text{J}$

That’s about 3.12×10¹⁵ J, which converts to roughly 745 kilotons of TNT (≈ 0.745 megatons). In short: a very large explosion — far above conventional ordnance and in the lower-range tactical nuclear bracket.

But this approach has several drawbacks, because the Sedan explosion was underground — the device was buried about 190 meters deep. The test took place in the Yucca Flat desert (Nevada), which has sedimentary soils with densities around 1,650 kg/m³ and volcanic tuff layers with densities in the 1,700–2,350 kg/m³ range — values that are lower than the rock and ground found around Sendai.

Because the target material in Sedan was less dense and the detonation was subsurface (which couples energy into the ground differently), the Sedan-based estimate understates the actual yield Gamera must have withstood. In other words, our Sedan comparison likely produces a conservative, lower-bound figure for the Legion blast — the real explosion that created Gamera’s crater was almost certainly more energetic.

Scaling the Fireball of Legion’s Detonation

Another method to estimate the explosion yield is to use the fireball diameter and apply the scaling law given in The Effects of Nuclear Weapons (Glasstone & Dolan, 1977). The visible fireball radius approximately follows the relation (a simple but robust empirical rule):

$R \propto Y^{0.4}$

where

Y = yield (explosive energy)

R = fireball radius

Rearranging for yield gives:

$Y = Y_{\text{ref}}\left(\frac{R}{R_{\text{ref}}}\right)^{2.5}$

(the exponent $2.5$ comes from $1/0.4$ ).

We will use these reference values:

Reference yield: Yref = 1 Mt

Reference minimum visible fireball radius at 1 Mt (for a surface contact burst, as in Gamera’s case): Rref = 430 m

Observed fireball radius for the Legion blast:

$R = 471\ \text{m}$

Step-by-step calculation:

Radius ratio:

$\frac{R}{R_{\text{ref}}}=\frac{471}{430}\approx 1.095$

Raise the ratio to the power

2.5

. One convenient way is to compute

(R / R_{ref})^{2} \times (R / R_{ref})^{0.5}

Square:(1.095)2≈1.200

Square root (0.5 exponent):(1.095)0.5≈1.046

Multiply:1.200×1.046≈1.255

Final result:

$Y \approx 1.255\ \text{Mt}$

So, using this empirical fireball scaling, the Legion blast corresponds to about 1.26 megatons of TNT (≈1.255 Mt) — a yield in the low-megaton range.

In the end, every method of analysis—whether based on crater volume, displaced mass, or nuclear fireball scaling—points to the same overarching truth: Gamera endured an explosion on the order of a multi-hundred-kiloton to megaton-level detonation. The crater’s immense size, the density of Sendai’s ground, and the fireball-radius comparison all reinforce that Legion’s blast was far beyond a conventional weapon. Surviving such an event not only showcases Gamera’s extraordinary durability during the Heisei era but also highlights the sheer destructive scale of the Legion threat. This moment remains one of the most compelling feats in kaiju physics—and a testament to why Gamera stands among the strongest defenders in monster cinema.