Question

it may seem like nothing is beneath antarctica’s two-mile-thick ice sheet.
a freshwater lake is hidden underneath it.
however therefore ← joining words
rewrite the sentences, and use one of the joining words in the correct place. your final response should be two sentences.

Explanation:

The two sentences present a contrast: the first suggests nothing is under the ice, the second reveals a lake. "However" is used for contrast. So we rewrite: "It may seem like nothing is beneath Antarctica’s two - mile - thick ice sheet. However, a freshwater lake is hidden underneath it."

Answer:

It may seem like nothing is beneath Antarctica’s two - mile - thick ice sheet. However, a freshwater lake is hidden underneath it.