  The prevalence of maximal extractable value (MEV) in the Ethereum ecosystem
has led to a characterization of the latter as a dark forest. Studies of MEV
have thus far largely been restricted to purely on-chain MEV, i.e., sandwich
attacks, cyclic arbitrage, and liquidations. In this work, we shed light on the
prevalence of non-atomic arbitrage on decentralized exchanges (DEXes) on the
Ethereum blockchain. Importantly, non-atomic arbitrage exploits price
differences between DEXes on the Ethereum blockchain as well as exchanges
outside the Ethereum blockchain (i.e., centralized exchanges or DEXes on other
blockchains). Thus, non-atomic arbitrage is a type of MEV that involves actions
on and off the Ethereum blockchain.
  In our study of non-atomic arbitrage, we uncover that more than a fourth of
the volume on Ethereum's biggest five DEXes from the merge until 31 October
2023 can likely be attributed to this type of MEV. We further highlight that
only eleven searchers are responsible for more than 80% of the identified
non-atomic arbitrage volume sitting at a staggering $132 billion and draw a
connection between the centralization of the block construction market and
non-atomic arbitrage. Finally, we discuss the security implications of these
high-value transactions that account for more than 10% of Ethereum's total
block value and outline possible mitigations.
