どんどん

どんどん is an onomatopoeic word that can have a couple definitions. Tobira states, "something that proceeds from one stage to another or continues to happen at a fast pace with no delay." With this in mind, it can mean things like "continuously" and "in succession", or it can also mean "steadily" and "rapidly."

地球温暖化のせいで、グリーンランドの氷がどんどん溶けているそうだ。
I hear that because of global warming, Greenland's ice is rapidly melting.

リーさんは怒って、ステファンの兄をどんどん打って、お金を取って出たんだ。
Lee-san grew angry, and he repeatedly punched Stefan's older brother, took the money, and left.

日食を見ているうちに、どんどん空が暗くなってきて、ついに真っ暗になってしまった。
While viewing a solar eclipse, the sky will steadily grow dark, and eventually it will become pitch black.

日本に来るから、私は食べ過ぎてどんどん太り始めたんだ。
Since coming to Japan, I've eaten too much and started to rapidly gain weight.