More ignorantism.

It's under a mile and three quarters of ice (3700 metres). It's in a cavity. When you drill into it, the static pressure is huge. Ice is just a little less dense than water, that's why icebergs float. Pressure increases as you go down under water, about 1 atmosphere for each 10 metres. So the pressure is 370 times atnospheric.

That doesn't matter much if the container is rigid, punch a hole in it and about a wineglassful of water comes out, and the pressure drops to atmospheric. Water is almost incompresible.

But Lake Vosok's roof is the ice bearing down on it. It's more like the piston of a pump. If you don't seal it off, it could squirt the whole lake, and it's precious content of three-million-year-old lifeforms (if they're there?) up into the Antarctic air and fall down as snow and ice.

And as the water comes out, the land surface should go down, like mining subsidence... but because water is resistant to compression, the difference in land level between water being liquid in the underground lake, and the new level with it frozen on top would be surprising- the level would rise because ice is less dense than water.

S they sealed it, responsibly, to keep it there.