This is a common situation in volcanoes, because when a volcano erupts, the huge heat will melt the rocks in the mountain, and if there is a lot of gas at this time, it will easily form a huge cavity in the soft mountain. If the impact of the gas is not enough to break through the magma, when the magma cools and forms rocks, the gas will be forever retained in the mountain, forming all kinds of strange caves.

The huge cave in front of us was obviously formed in this way. After all, the huge crater is just a few meters away.

The top of the cave was thirty meters above the water surface, and Yang Jing could not see the end of its length and width. However, the main submarines of the Japanese Navy in the middle of World War II were about one hundred meters long, and this large pool for submarines to enter was at least one hundred meters long.

The width of this large pool is about 15 meters, which is about the same as the width of the gate. The widest submarine of the Japanese army during World War II was the U142 submarine technology introduced from the Germans and built by themselves. This submarine has 1970 tons (above water)/2791 tons (underwater) and the maximum width of the hull reaches 9.22 meters, which is the widest submarine of all models of the Japanese army.

Therefore, the 15-meter-wide gate and pool are large enough for Japan's widest submarine to enter safely.

In this huge hole, except for the slipway which was built of reinforced concrete, almost all of Yang Jing's sight was the most primitive rock wall. Obviously, this secret base was a natural cave that Onishi Takijiro accidentally discovered, and then slowly transformed into such a secret underwater base.